My problems began shortly after a Dec 30, 2020 Roku update. You can check your last update to see when it was done in your Roku settings (to see if it was updated sometime after your initial installation when it was working). My rarely used older laptop still worked on the wireless just fine, and the Roku is the only wireless device I normally use with everything else hardwired for security reasons.
If it was working before it quit, then your modem/router was obviously broadcasting on 2.4 gig and it is highly unlikely that it changed itself during use but that should be verified anyway. Your ISP should be able to tell you how to access your modem/router if you rent it from them (generally you just access it with a browser and login using your username and password) or if you own it yourself the manufacturers website will give directions along with all sorts of instructions available on the internet.
So first follow all the standard stuff from Roku and it that doesn't work then do them over and over and over again till you're satisfied you'll keep getting the same results every time then attack your modem settings to try a few things.
When all that standard stuff fails there are two things you might try that finally led me to a workaround solution. Not a real fix but it works and I can use my Roku.
First try setting your network to use an non encrypted mode, just turn encryption off, to see if that works for you (it did for me). If it does that means the fault lies in Roku (took several hours online with my modem technical support trying every possible setting to determine this).
Second, since no encryption is not a good idea, this is worth trying since it worked for me. Set up a guest network in your modem/router settings and see if it will log onto that. That allows you to use the Roku even if it will not log on to your normal home network.
FWIW, when it became obvious the problem was with the Roku I first thought the old on had just gone bad and bought a new one. It updated itself after initial connection and the problem was the same with a brand new one so I don't recommend that as a solution. It's sad that Roku has no real technical support the way almost everything else does.
