Please see Connection Troubleshooting Tips. If you can't find the problem, call them at the phone number on your bill or 1-800-Comcast, or chat with them using the Internet Support option at https://support.xfinity.com/chat. If they can't fix the problem remotely, insist they send a tech out to identify the cause and correct it.
Loss of Internet service is often due to a poor connection between your equipment and Comcast's network, usually in or near your home. Since you've already swapped out the modem and checked the cables, you'll probably need a tech.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast rental device, or anything outside your home, you shouldn't be charged.