AC1200 C6220 WiFi not broadcasting

I doubt I'd buy the support contract either. The cost of a contract isn't much less than a new C6220. 

And if it still works while wired it, you could always just get a router and put the c6220 in modem only mode and just use it as a modem. 

You've tried most of the things I'd recommend like the holding wifi on/off, factory resetting. 

The only other 2 things I'd try. 

1. have comcast re-provision it. If something corrupt in the firmware, it needs to be overwritten. And there's no way a home user can do it on that model. So the isp would need to reprovision to do so. 

2. disconnect everything (even coax from it), then while powered on, do the factory reset. Push and hold reset up to 30 seconds while powered on. 

If neither of those works, I'd investigate just buying a router and using the c6220 as a modem only device.