Good luck. Comcast has been messing this up for 2 years, and has apparently started pulling the plug on everyone else who in the past was able to do this (they admittedly somewhat need to do this for pods, but probably could make it so they didn't have to). Notice how many more threads about this have shown up recently? 2 years ago no one knew what us few were talking about when we started yelling about it, now it's coming up more often.
The folks you get at Comcast are clueless as they spout off just the info they see in their knowledgebase screens using outdated information and 90% of those people have no idea how a Wi-Fi network even operates. IF you are lucky you might be able to get a Level 2 or higher (do they exist?) who at least knows what you are trying to do and in an even greater longshot they'll be able to help fix it, but I wouldn't hold my breath. Might as well just start shopping for your own router and modem now and get it over with. It's a shame because in many ways their hardware is pretty good, but like the Comcast of old, they seem to find a way of messing it up in the simplest of ways.
They say there's a feedback mechanism for these kinds of problems...but it's a blatant lie and they don't care about it at all. It's a SUPER EASY fix with ABSOLUTELY ZERO reason to be this way.