Model numbers would help, but I’ll take a guess: Some of Roku’s most basic models use 2.4GHz Wi-Fi only. Some Routers (especially ISP rental routers) have been known to get automatic firmware updates that turn off 2.4GHz. So, you need to login to the router and turn it back on. Another more subtle problem is that some routers get an update that turns b mode off by default – ie: you’ll see wireless modes g/n enabled but what you want is b/g/n.
Banned but back. Because why not?