The Guest Network feature will not work correctly unless the AirPort Extreme is set up to provide DHCP and NAT routing services.
Or, another way to say the same thing would be that the guest feature will not work correctly when the AirPort Extreme is set up in Bridge Mode.....since the Airport Extreme is not acting as the main router in that type of setup.
If the guest feature is important to you, you might be able to configure your Xfinity modem/router to work only as a simple modem.....or.....swap the modem/router that you are using now for a simple modem.
If that is not possible, you might consider using the Xfinity wireless as your "guest" network and the AirPort Extreme as your "main", or "private" network.