Roku Express/Express+ (not the relatively new Express 4K/4K+ models 3940/3941) and Premiere/Premiere+ (model numbers above 4620) are lower end units with single band Wi-Fi radios that only support the 2.4 GHz band. They, and many other devices of all sorts that are 2.4 GHz only, can't even see the 5 GHz band, let alone connect to it.
Several things to check relative to this.
If you have one of these Roku models and a dual band router with both 2.4 and 5 GHz bands, be sure the router's 2.4 GHz band is turned on. There have been reports lately of updates to the router shutting off the router's 2.4 GHz band, especially with equipment supplied by internet providers. Users with no access to their router's settings have had to contact their provider to get 2.4 GHz turned back on or in some cases have the router replaced with a new one.
Within the 2.4 GHz band, Rokus support channels 1-11 as are used in North America. If your router supports 2.4 GHz channels outside this range make sure your router is set to use a channel in this range and that you aren't using an auto setting that could select channels outside this range.
There also have been reports of router updates changing the router's 2.4 GHz connection parameter settings. This has been reported with equipment from Xfinity, AT&T, and Cox, but there may be others. Affected users report the router configuration needs to be switched from g/n protocols back to b/g/n. If you don't have access to your router's control panel you may have to have your internet provider do it for you.
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