You are SOL.... That Comcast Router will not work with a WiFi network extender.... Sorry. I actually got out of a Comcast Tech that they do not allow range extenders. You can either run a wired connection to an Access Point or (what I ended up doing for a customer) Get a real WiFi Router, put it in the DMZ of the Comcast POS and then add your Extender to the regular Router.
Alternatively, take the POS back and get a new Arriss Modem and then add your own router with enough power to cover your home. (What I did for myself)...
Get yourself something like the D-Link DIR-855L and be done.