DIAGNOSIS This is usually a double NAT conflict or an IP address clash. The Xfinity modem must be fully placed into bridge mode, disabling its routing functions, before the RAXE500 can manage the network.
FIX First, confirm bridge mode on the Xfinity modem. You may need to contact Xfinity support to enable this. Once confirmed, connect the RAXE500's WAN port to port 1 on the Xfinity modem. Power cycle both. On the RAXE500, log in and go to Internet > Internet Setup. Ensure 'Get Dynamically from ISP' is selected. The router should now pull a public IP directly from Xfinity.
IF THAT DIDN'T WORK The RAXE500 may have a conflicting default IP range. Temporarily connect a computer directly to the Xfinity modem. Check the modem's gateway IP. If it's 10.0.0.1 or 192.168.1.1, you must change the RAXE500's LAN IP. On the RAXE500, go to Advanced > Setup > LAN Setup. Change the IP Address to 192.168.2.1, then Apply.
HEADS UP In bridge mode, all Xfinity advanced features (like xFi pods or parental controls via their app) will stop working. All management must be done through the RAXE500's interface.