@PugNugget 

Router mode is what you need, that is to replace the Xfinity Wifi. You'll also need to have Xfinity to BRIDGE the Modem/Router. What that means is TURN-OFF the Wifi function of the combo modem/router. Otherwise you'd have competing wifi signals and have problems like interference. You can't re-use the SAME SSID's as the Xfinity had either unless they bridge the modem (only sends the WAN cable to the router, no Wifi).