I read this and wonder if the following was attempted.
1. Find the "lease time" and double it. This is nothing I can tell you more about as the router is unknown.
2. Try another router. I think you are only using ATT's gear but to nail if it's ATT we would have to test with our own routers.
We have AT&T Fiber, 100 mb/s, which we have had since December 2016. We absolutely love it, it has been fast and consistent, up until sometime this summer. Around July, our 5ghz band started disconnecting and then reconnecting every single day (usually once, sometimes multiple times throughout the day). It is still happening 3 months later. We have had 7 AT&T technicians come to our house, replaced the fiber cables, the modem/router (twice), changed our IP address, etc. Every single day (at random times), the 5ghz band will drop out, and all the devices that are connected to it will lose internet. After about 2 minutes, it comes back. The devices that are connected to 2.4ghz don't lose internet. It is so frustrating, as we work from home and it has become quite annoying. We don't know what else to try. We've changed the channel on the 5ghz network multiple times, and also left it on auto. We've combined the 2.4ghz and 5ghz bands, and also separated them. When the 5ghz band goes out, all lights on the router/modem stay green and flashing appropriately as they are when it is working.
The current devices connected to the 5ghz network are - 2 laptops, 2 Galaxy smartphones, an iPad, 2 Apple TVs, and a DirecTV box. The devices connected to 2.4ghz are a Roomba and another laptop.
Any thoughts on this matter would be appreciated. We keep considering switching back to Comcast, but we are afraid it's not even an AT&T issue at this point and that it would happen even with Comcast. We have no clue what to think now after all the things we have tried to fix it.
Thanks in advance.