On the side of your Xfinity modem/router will be a printed label with info showing a URL or IP address that you can use to access the setup menu. But this is only necessary if you're trying to access the settings. Odds are very, very unlikely that the default security is setting WEP anyway.
That label will also include the SSID (network name) and password of the default WiFi network. That's the WiFi network you should be trying to set up on your phone, using the indicated password.