After looking into it some more, it appears you are not even bound by ATT to use that router (assume you are renting it). Some ISPs require you to use their own. Looks like plenty of routers compatible with ATT and fiber. Changing my vote to ditch that router all together, save on the rental fees, and buy a fiber compatible, configurable router that allows you control over your network.
I was mystified by this same problem and learned something which I will post here in case it helps someone.
ASUS routers have settings for the control channel to be used for wireless (both 2.4 and 5 GHz). This allows one to pick a specific channel from a list or to select "auto." When selecting auto there is a checkbox for "Auto select channel including DFS channels." I had that checked. By unchecking it, all went back to normal (after waiting for things to settle down). Perhaps other routers have a similar setting.
What had me really confused at first was that all the other 5GHz devices in my house were working fine -- as was the Roku until it suddenly stopped working. I must have checked that setting at one point and, perhaps, existing connections were unaffected until there was some sort of reset.
Honestly, I'm moving away from Roku after many years just because of this issue. I recently upgraded my wifi with Eero because I just don't want to tinker with networking at home anymore. Of course, in a crowded neighborhood, Eero chose a DFS channel outside of Roku's vision. My network is running great, but my Roku's will now only connect to 2.4ghz, and on one in particular, it's just slow enough to be problematic. I'm moving my streaming to a WiFi 6 enabled solution. It sucks because I love Roku, but their poor network implementation needs to be fixed.
It should be pretty easy to select a non-DFS 5GHz channel in your Wi-Fi router. That should at least hold you for a while so the need for a Roku replacement isn't as urgent.
Unfortunately, that feature isn't available in Eero. It was on my old router. Not ideal, but be that as it may, Roku should support it. The fire stick does.
There's valid reasons to NOT support DFS channels. In some locations, they're not permitted to be used.
And right about that as well. Roku could explain why 5GHz has been disabled instead of just refusing to connect by ignoring the 5GHz signal.
I guess the real surprise is why the router gets into this precarious situation with no way out.
Agreed. But that's generally turned off at the router. In fact Eero actually disables it for affected regions. I mean I don't apologize for Eero's lack of configurability. But, IMHO clients should be able to use the channels if the local network supports it.