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.