I bought a new Roku Express 4K to replace an older Roku device. My home network appears, but when I try to connect I get "Checking wireless connection" with a red X and "Unable to connect to wireless network." All other devices are connected to the network. I've restarted the router (Arris Model TG1682G with both 2.4 and 5 GHz) and Roku device (Model 3941X). I've checked and entered my Xfinity password many times, and also tried the password on the router.
You need to enter the WiFi SSID password - not your Xfinity account password or the modem/router/gateway management/access password.
If you are unsure if you are entering the correct password, check the saved WiFi password on another device that is successfully connecting.
Your 2.4Ghz and 5Ghz bands should have separate/different SSIDs, though they can use the same password. Make sure you disable any "B" mode compatibility (G/N/AC only), limit the bandwidth to 20Mhz, and manually configure 2.4Ghz to channels 1, 6, or 11 and 5Ghz to 36 or 149.
If none of that works, try using your phone as a HotSpot for the initial/setup/download/activation, and switching to your broadband WiFi afterwards.