I have a twofold problem. I lost my roku remote (have been using the official Roku app on my android phone), also it does not see the new 5g network in my house. I've tried unplugging/replugging the unit to no avail. The official Roku app does not seem to have the option to change networks.
Regarding the inability of the Roku to see your 5ghz network. Verify that that the settings for your 5ghz network are configured to use a wireless channel of either 36-48, or 149-161, rather than "Auto". Could be that your router switched to a DFS/Unsupported channel that the Roku cannot see or connect to.
Using the Roku mobile app, you should be able to use it to switch your Roku to the 5ghz band by navigating through the Roku menus to Settings..Network..Setup Connection. However, the option to switch to your 5 ghz network will only appear if you have the 5ghz configured correctly.
Try restarting both your Roku and your router, and then login to your router's admin panel to make the necessary changes to the 5ghz wireless settings.
Feel free to post back with your Roku model number, router model number, and ISP if you need further help.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102