AT&T Fiber's routers have, in the past, had a tendency to overheat, offered false promises like "DMZ Plus" mode and have had a host of issues that led to a black market of people selling stolen AT&T certificate files [0] on the internet so you could bypass them, because they use 802.1x between their "Router/gateway" combination device and their ONT, when they're separate devices. The AT&T XGS-PON network is mostly coupled now, which has led to another group of people now creating compatible SFP+ modules to replace the entire GPON stack because of this.

I could be wrong, but I think AT&T Fiber is the only US ISP that doesn't even allow you to directly connect to their network. If you use any of their provided routers, they only offer "DMZ Plus" mode that still leaves their router/gateway managing state tables, which is vulnerable to hardware and software issues from the ISP. This leads people down the path of programming SFP+ modules and spending a lot more time than they should have learning about ISP networking, just to have a safer router/modem.

[0]: Due to security issues in the router/gateway firmware, various people have published guides and/or run actual businesses shucking routers/gateways from AT&T by exploiting them, grabbing the certs and private keys, and then re-selling them to people who need them. These don't get you free access to the internet or anything, they just let you authenticate to the network with your own device.