Connecting to 5 GHz Wifi - Roku Stick+
The Roku will never connect on 5 GHz as long as the router insists on using a DFS channel. My suggestion is to add a separate access point on your network for 5 GHz. I have a UniFi WAP as one of my access points. By default it too uses DFS channels, so I manually assigned the channel and all my 5 GHz capable Roku players can connect. I didn't need to add it for the reason you need to, but it's still a valid choice.
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.
