I don't usually use a modem-router combo because they inevitably crap out in weird ways. What I usually do is buy a cable modem, then wire it into a pfSense firewall, and pull one or more wireless access points off of that. It's easy to troubleshoot and I never have problems with it. The setup I have now covers a quarter acre with a single access point.

Comcast has gotten a lot better about supporting customer equipment. They maintain a "supported device list" and as long as you buy something off that list they'll help you. Granted if you're buying your own device you should be prepared to do some minimal troubleshooting like using ping and looking at LEDs.

I once came home from a business trip to no internet. When I called Comcast I explained the troubleshooting steps I took and what the modem was reporting. They sent a tech out in 2 days to tone the line. When they couldn't get a good tone they sent a level 2 tech out to replace the line. Turns out someone had put a screw through a cable chase when they were re-siding the building while i was away.