Specify the issue? It's the same issue everyone in this thread is having.
It happened to both my Rokus. One is a TCL Roku TV and the other is a Roku 4K Streaming Stick.
My ISP is irrelevant.
Of course I've restarted everything. And, of course, the power outage restarted everything.
Yes the devices will connect to other WiFi networks. They'll connect to my same existing router if I change the SSID.
As has been explained in detail by your users here in this thread and many others, the problem is in the Roku software. It seems that the Roku software gets stuck on a wifi channel, and if the channel changes it can't see the wifi network any more.