The Internet and the router are digital signal things. Am I wrong to think they don't introduce noise into the bitstream?
Now if a power supply is near some analog device in your setup, the solution would seem to be putting some distance between them or a similar action.
We never had Comcast. On AT&T and now our superior local ISP, Sonic Fiber, I have used a cheap TP-Link Archer 8 modem. It works fine. AT&T requires its gateway device, so my router was behind it as an access point. Sonic lets customers connect their own router to its ONT gateway.