I have Xfinity internet service that's about a year old. In the last few months, I've been unable to reach certain web sites reliably. It was only after working from home and being unable to connect to my company's VPN, that I've figured out this is due to WiFi clients not getting a proper IPv4 address, and therefore being unable to connect to anywhere that uses IPv4 only.
The gateway router in question looks like it has the part # CGM4140COM
Looking through the archives of the forum, it looks like someone named @ComcastJohnN was able to resolve this for people in the past.