Fellow customers: Got a nonbranded, unlocked phone? Report here on how it works with AT&T

Exact make and model of phone (different models have different frequencies -- mostly an issue with Huawei).  We have or had: Nexus 6, Nexus 6p, Samsung GS6 International, Moto pure edition, Honor 8, Google Pixel, Google Pixel 2.  I have a Razer in the house, and Oneplus 5t on the way.  
Whether you are on ATT Prepaid or Postpaid.  Postpaid, 4 lines.
Your version of Android (in Settings / About Phone or about Tablet). Either android 6, 7 or 8. 
Where you live. (note: reception varies block to block -- great speeds in your town doesn't guarantee service at your house -- on any phone, supported or not).  Granby, CT.  Towers are mostly LTE band 17.  Reception varies from lousy-122 to excellent -85 in town.  The Google phones have better antennas and pull signal where others do not.   Tower speed at Home is just under 10 Mbps.  In town it’s 35 Mbps. 
Optional: Frequency Bands Your Phone Uses.   Too many to list, but all covered the ATT list.
(For the convenience of those reading this thread, a chart of ATT's current operating frequencies is below).

I had trouble getting LTE to show in the notification bar with the international Samsung. All the other phones auto registered on ATT. The Honor 8 and Google Pixel 2 get HD voice and work with number sync.
I am expecting to have trouble with both the Razer and the oneplus. Since the oneplus was introduced the work around is to have a cheap phone or old phone IMEI to use as a dummy in the system to get LTE. I have a broken iphone for one IMEI, but may have to pick up another phone.
I will write my review on the Xiaomi Mi Max 2 below. Thanks for participating!

  Good luck with that.  There are several brands we don’t recommend because they don’t get LTE even with the work around.  Maybe you can figure it out.

REVIEW: XAOMI MI MAX 2 on ATT Network -- UP AND RUNNING LIKE A CHARM

Xiaomi Mi Max 2, 6.44 Inches, Dual Sim, Unlocked PhoneMake: Xiaomi  -- Model: Mi Max 2 -- Year: 2017. -- Type of ATT Account: Prepaid.
Android Version: 7.1.1 (Nougat) -- Living in: Branford, CT

Device Operating Frequencies (Bold frequencies are compatible with ATT):

To be honest, I didn't have high hopes for connectivity when I purchased this phone. As you can see from the table above, only one 4g Band -- LTE 5 (850) is supported, plus Bands 2 & 5 in 3G HSPA. But I am a prepaid customer, so my choice of supported devices is strictly limited. I'd been a lifelong Samsung customer and used to admire the build quality of their phones, but I'd had two different models in the tad over a year I've been with AT&T and they were both shoddy. Worse, when they did break, support was nonexistent.

I determined to get a better-made, non-Samsung Android phone with at least a six inch screen. A Huawei Mate configured with all supported AT&T bands would have been optimal, but it was over my budget.  Xiaomi's stunning flagship phone, the Mi Mix 2, was somewhat less expensive, and it's configured with enough bands to get coverage on any network anywhere, but I would no more buy a phone without bezels than a car without bumpers, no matter how beautiful it might be.

Whereas the Mi Max 2 with nearly the same specs, a larger screen,and bezels, cost $229 total including international shipping, with the Global rom pre-flashed. I decided to take the chance and I am so happy I did. 

I inserted my micro SD and it worked immediately -- Plug & Play. The IMEI updated automatically to reflect the model I bought -- which has 4 Gigs of ram and 64 gigs of internal storage. Hot-spotting works. Volte works -- it didn't in my ATT Samsung. Not that it does me any good, since Wifi calling is not included in the enhanced features for prepaid customers.

But here's the miracle -- I get much better reception and speeds on the Mi Max 2 than I did on either of my two Samsungs.

Granted, this may have something to do with my neighborhood. On the Samsungs, I almost always connected on LTE Band 2 4g -- which isn't actually 4G but HSPA+. (The United States is the only country that permits carriers to list 3g HSPA+ speeds as 4G). As far as I can tell, I am connecting on the exact same bands, except now it says H+ in the upper right hand corner of my display instead of 4G. My download speeds are faster -- that, I have no explanation for.

A lot of Mi Max 2 users from the forums say they're getting 4g confirmed on their display via Fdd-LTE Band 5, which ATT has rolled out in a lot of areas, but there aren't any Band 5 towers around here, so I can't confirm that. I'll check it out the next time I am in Manhattan and post accordingly.

In the meantime, my guess is the phone is faster because even though it didn't cost more, Xiaomi is a value-brand with terrific build quality that provides many high end features at a budget price.

The CPU is Qualcomm Snapdragon 625 (MSM8953), Cores: 2.0GHz,Octa Core GPU: Adreno 506, with 4GB RAM and 64 GB of Rom (You can get more -- mine was entry level).
I'm not a gamer, but my gamer friends say it is more than adequate. The speakers are great, incidentally.

Which is not to say it has every single little thing on your wish list. No wireless charging. And the OTG, which works admirably in every other respect, doesn't support video out. You have to screen cast. (I'm a hardwired kind of girl -- cables matter to me, so I'm hoping for a workaround at some point).

Oh -- I forgot the real bonus! You know you're not on Samsung when your phone isn't preloaded with crap-ware. I had a few little worries about Xiaomi apps phoning home every two seconds to report my activities back to China, but as it turns out, the Mi apps (Mi is shorthand for Xiaomi) -- unlike Google Apps -- use few resources and don't phone home without permission. Also, you can disable them. Normally, rooting is the first thing I do, but I'm not sure I need to with this device.

But I do suggest if you buy this phone that you spring for t with the Global Rom preloaded (especially if the difference is only the $10 I paid). You won't have a problem either way -- the Global Rom with Google Apps Preloaded is a fast, free download from Xiaomi and flashing it is simple with their built in updater app.

Still, it was tremendously reassuring to just stick the Sim card in and be more or less done, apart from customizing the home screen to your liking. Note that out of box, looking at the screen doesn't make you go 'Wow.' 

I was used to Samsungs, pre-loaded with the most colorful, 3D wallpaper and icons possible so brilliant they're like a magic mushroom experience. 

Mi Max 2 -- well, it launches on screensaver and looks more like Win 98.  But there are all kinds of beautiful Xiaomi themes, wallpaper and icons available for free (really free, no ads, no in app sales) on Google play and perhaps in Xiaomi's own 'app vault.'

I have yet to explore it, because they left app vault out of their most recent Android update, in favor of rushing us fixes to minor 'bugs' reported by users. Another plus -- when they rush out updates, unlike Google, they actually wait until you've granted them permission to install!

(Of course, unlike Google, their revenues come mostly from hardware, not software ... but still).

Anyhow, yes -- I LOVE this phone. And it would be great to think that at some point Xiaomi would have enough of a presence in the United States that AT&T might include the brand in its  its lineup and shake up Samsung's game!

 Mi Max 2 specs:

Body

Display

Platform

Memory

Camera

Sound

Comms

Features

Battery

Misc

Tests

Thank you for your report, this was tremendously helpful. I knew about the spare IMEI number to mask what phone you are using but haven't bothered, since I'd be on Band 2 either way (in my area it's HSPA+, regardless of what they call it).

You are clearly a lot more sophisticated about this stuff than I am -- this was my first unlocked phone.

One thing I have been wondering. Do you get them to keep the 'trick' imei by asking them directly? Because my imei updated all by itself (I used a card from an ATT activated Samsung).

There are also apps that change what IMEI appears, much like you can change the User Agent on a web browser, that supposedly elude this automatic updating. I don't know how effective they are or whether they still work with Nougat.

Two guys in the XDA developers Android forum were among those reporting they got Band 5 LTE consistently on ATT -- I'll check back to see if they're still getting it. In the same forum, developers had a method to 'unlock' new bands on the Mi Max (the model before Mix Max 2). They succeeded with all bands except for LTE Band 20 -- the hardware wouldn't support it. If you are not already familiar with that forum, I'd check out the threads on the phones you use in your household. Offhand, I think there were similar threads on the Oneplus brand.

BTW -- you were able to use an Apple as a dummy for an Android phone? I never would have guessed that was possible!

@brasscupcakes

First, I take no credit when it comes to technical stuff.  I learned a lot on this forum from my betters.   I do have a good memory for tidbits of information, which means I retain well what I learn.  

   The IMEI will update once you move your SIM card.  But to get LTE on some phones the work around does require a call to ATT and request ‘Advanced Tech support’.  Enjoy the phone tree if you do 😱.  It’s terrible.  

   I didn’t know there were apps to change the IMEI, SHARE!  Maybe it will save me a call when we get the Razer running and the 5t arrives.  

   The XDA forums usually is just a bit above my head.  I won’t pretend I go there as a first or last resort.  😆😆

   I don’t know if I can use the Apple IMEI on the android, haven’t tried it yet.  It’s the only broken phone I have with a valid IMEI.   If not I have to find a couple cheap androids.   Bummer.

Nice review.....

We get asked about this phone often and always say 3g (HSPA) only, which is still accurate.  Some don’t care, like yourself, and get it anyway.    This is a good reference for the next person.  

There are are some other decent phone options that will get LTE in the USA.  The larger Best Buy stores have a big display of unlocked phones.  Full range of sizes and prices.   I’m always tempted.  I really need a 12 step program for phones.  😂

I wish there was an app where you could change your IMEI to anything you want.  That way you can avoid the pain of getting ATT to add an IMEI manually. 

The knowledge must be out there. Once I am strong enough with the dark side, I'll be able to do this. 

I am back to using my Moto G4 Plus which I purchased direct from Lenovo.

I live in rural SW Alabama and it picks up pretty good as far as reception.  I get LTE on band 12 with android 7.0. No HD calling.  It drops to 3G when I makes calls. 

@Anthony.Dukes   May the force be with you...

@Anthony.Dukes

Spoofing a imei is illegal.

How is spoofing an IMEI illegal? 

https://www.congress.gov/bill/112th-congress/senate-bill/3186/text

Have read.  There is a reason imeis are granted from a organization.