Use my own modem?

As far as AT&T's fiber service goes, you must use AT&T's gateway, period. However, you can use a third party router for better coverage if you put the gateway in pass through mode and let the new router handle the routing and WiFi duties.

Comcast (Xfinity) allows customers to use their own, approved modems and routers without paying a monthly equipment fee.

@Jormungandr 

You cannot use your own Gateway/modem. No way around the fee.

You can use your own router using passthrough mode, but the modem part of the Gateway is always AT&T's.

I’ve seen some posts that Att may have allowed original DSL product customers to buy their own gateway.  IIRC, the last message I saw on this said the practice was discontinued in 2015 and was from an Att technician.  

I started as a customer when Uverse product was rolled out to ex BellSouth accounts circa 2010 using ADSL technology.  All gateways from that point forward had a mandatory fee. This includes customers of VDSL2 and Fiber.  

There are cases where Att will waive the fee, but the use of one of these BGW gateways remains mandatory.  It’s just part of how Att designed their network, like it or not.   I can tell you not all Telcos worldwide follow this practice.  For instance, have a friend who lives in London that can buy his own router for use on his British telecom service.  

As for the comparison with Xfinity product, it’s a little worse actually for customers on a higher speed plan if leasing a gateway.  Comcast states they offer gateway design updates at a more frequent rate than Att.  For instance their latest gateway supports WiFi 6E and customers can get upgraded tech every three years.  And as was already said, you can use your own modem provided it’s on their authorized product list.  The bigger cable companies are in the process of upgrading their network for DOCSIS 4.0 to offer symmetrical or quasi symmetrical speeds to compete with fiber.  

@gr8sho  We have Xfinity, 800/20. And I typically get about 950 down and 25 up, which is more than enough speed for us (we are not gamers) for 2 home theater systems (hard wired) and about 20 WiFi devices at any one time. I use an Xfinity approved Motorola DOCSIS 3.1 modem and a Netgear AX4200 mesh WiFi system (WiFi 6) with two hard wired satellites. The downside is if something goes wrong with the modem, which, knock on wood it hasn't, Comcast won't do any remote troubleshooting because it's not their product. Comcast is working on DOCSIS 4.0 but I have no plans on upgrading to that level because we just don't need it. And I have to say, even for Comcast, the service has been reliable and fast. Support, when I've needed it,  could be better but it's not as problematic as AT&T Support is. And, there's no monthly equipment fee (it's probably baked into the plan 😉). 

Yes, @OttoPylot , I also have Xfinity service at another location, which service unfortunately isn’t as reliable as Att has been, but their XG3 gateway has never given a lick of trouble.  So it’s not impossible for an ISP to have a reliable gateway with proper WiFi.  My neighbors in my development do not complain openly about Comcast.  

Xfinity also has much higher headroom than Att for bandwidth, which presumably is their way of conceding their service is shared bandwidth, my guess.  

If Comcast beats Att with fiber in my development with DOCSIS 4, I’m probably gone as a customer at that point.

@gr8sho  Yeah, Comcast was our first foray into cable services. Before we moved, we were strictly an OTA HDTV/streaming family so we had no need for cable. But in our new location, OTA HDTV is just not an option so we had to switch to Comcast being as they were the only game in town other than Frontier Fiber (which is now Ziply in our area) and the reviews for Frontier were worse than Comcast. But, we've had no issues with Comcast. So far so good. Comcast (Xfinity) does seem to provide about 10% higher dl/ul speeds than posted so shared or not, it makes no diff to us because it is plenty fast enough for us and is very consistent and reliable. So I guess it comes down to luck and location. I do like not being tied to a providers equipment tho because generally speaking, customer provided equipment is better than ISP-supplied.

Is because black rock joined AT&T to make gigapower and they need control of your modem to have access to all device connected to there modem 

Um, not exactly.  But AT&T does want to be able to have a device they can control in your network for network management, testing and diagnosis.  So, AT&T technically requires that you use their Gateway.