Asus Zenfone 2 ZE500KL stuck on H+

Following up to the following forum topic, there is a similar odd problem on my Asus Zenfone 2 ZE500KL (Worldwide version) in that it seems to be stuck on HSPA+ (H+) network after the recent purchase of a Go Phone SIM card.

A little back story: this phone was purchased in France as I was living there (and where I had an unlimited text/voice no-contract plan with 5GB of data in 4G LTE for about $25, how about that?) and I recently moved to the US where I had the immense pleasure to learn that if one wants a bit of data with a decent carrier, you'd need to pay tremendous amounts of money. I was no-contract back in France and there is no-way I'm getting tied up for two years with a carrier. Given the Asus Zenfone 2 ZE500KL seems to be working on T-mobile and AT&T, I decided to give a shot to AT&T for the coverage (I live in somewhat rural Pennsylvania). I took my device to the AT&T store (not in rural Pennsylvania) and before making the big leap, I asked if I could check with them if the device would work on their network and also if it would be working with 4G LTE. The rep goes in the back, comes back, and we put an already activated SIM card in my phone, just to try, boot it and it works great, he says I get 4G signal and that GSM signal is great. Awesome, sign me up for the $45, 2GB, GoPhone plan!

Now the rep activated the SIM card for me and I was good to go. This is where it went south: after having booted up the phone, it now only says I pick up H or H+ which is odd to the salesperson who is witnessing it too but he sends me home saying it will pick up the 4G signal. I don't question it, after all, we tried and he saw in front of his very eyes that MY phone CAN pick up 4G LTE.

Problem is, it HAS been stuck on H+ since my purchase and activation of my SIM card. I called AT&T and was on the phone with the technical staff for a bit and they first dismissed it as "your phone is not an AT&T approved phone and it MAY NOT pick up 4G LTE" which I understand but why does it show H+ at the same location, in the same store where my phone displayed a 4G signal with an AT&T SIM card? The person on the phone responded with : "you know HSPA+ is 4G". Fuffff, only AT&T considers H+ as 4G. H+ is 3G++ at best not 4G at all. The person on the phone eventually folded and apologized that the salesperson at the store got my expectations up and that I would be stuck with H+. This is where the conversation ended, the person was nice and I didn't want to be rude and insist that something was up.

The Zenfone 2 ZE500KL that I own has the following network specs (according to ASUS website):

WCDMA/TD-SCDMA/TDD-LTE/FDD-LTE
Data Rate:
DC-HSPA+: UL 5.76 / DL 42 Mbps
LTE Cat4:UL 50 / DL 150 Mbps

3G :
WCDMA :(WW version)
850MHz(5)/900MHz(8)/1900MHz(2) /2100MHz(1)

4G :
FDD-LTE: (WW version)
2100MHz(1)/1900MHz(2)/1800MHz(3)/850MHz(5)/2600MHz(7)/900MHz(8)/800MHz(20)

I know AT&T uses band 4 and 17 for 4G but I am REALLY wondering why in the name of technology did my phone pick up 4G signal in the store with the store's SIM and only picks up H+ when my activated AT&T SIM is inserted when at the very same store. I am aware that my phone is just GSM-unlocked, that it may not work with certain networks, but I certainly didn't dream the 4G signal pickup.

That would be great if an AT&T technical rep would help as to why I'm experiencing and that would be great if there was a fix for that (I tried modifying the APN settings, without any success). Otherwise, I guess I'll be stuck with H+ which would be alright.

Thanks in advance,