When you say "though Comcast", does that mean using their network, or it is their device? If it is their device, contact them and scream until they make it work.
If it is yours, then you may need to correct administer the DNS in the router. If it worked good for your other ISP, now it doesn't it is becuase COMCAST network does not like to look at the other providers network resources for routing. Change the DNS is the router to point to the MODEM, then to 188.8.131.52
Give that a try, and let us know!