Do I need to buy a Comcast router or can I use Cisco?

Your cisco router has nothing to do with it. they are trying to con you into paying more. the router plugs into the modem and the router makes the wireless signal. I am not sure if comcast has modems or not as I have not used cable internet, but if it does use a modem, then your cisco router should work just fine. if not, you will have to buy their router.