I’m going to guess that the Comcast WiFi has only current encryption methods, and not the older kind, and that your laptop might only be able to handle the older kind, it’s possible a driver could help but encryption might be done in hardware for speed. If that’s the problem and nothing else works, you can get a cheap WiFi ‘access point’, plug that on a wire into the hib on the comcast, and set it to the older style encryption then connect your old laptop to that.