Please provide the model number of your Roku device. Regardless of the actual Roku device, they are all capable of at least connecting to your 2.4Ghz band of your network.
Simple troubleshooting is to restart your router and your Roku device. Sometimes multiple restarts of the router. You can also try after restarting your router w/ no success to reset the Roku network connection. (Settings..System..Advanced System...Connection Reset...Reset Network Connection). When you navigate back to the Connection menu, press SCAN AGAIN a few times refresh the available connections.
If still no success, then you will need to login to your router and verify that your 2.4Ghz network band is configured properly and broadcasting to use Roku. (Under 2.4Ghz Wireless Settings, verify that the Radio is Enabled, SSID is Enabled/Visible and Broadcasting, wireless channel is set between 1-11, mode or protocol is g/n or n, and your channel width/bandwidth is 20Mhz).
Please post back with an update or if you need more help, please include Roku model number and your router model number.
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