First and most obvious, have you tried turning it off and on again? (Sorry, IT Crowd was on last night) but seriously, when you plugged the router into the cable modem, you power cycled the modem and then the router, right? I've noticed that some cable or dsl modems get hung up on one MAC address until they're reset so moving the cable from one device to another without power cycling can be a problem. If you already did that, try going into the web interface for your router and look for a button to "release and renew" your external IP address or "WAN" Ip address. XFinity probably assigns a static IP to your modem's MAC address that may be managed via a dhcp server.
Big Dawg #1408, TT #1408
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