Internet disconnects intermittently

I used Xfinity Internet for more than 2 years since I moved to Vancouver, WA. The Internet service was always good until May 2020. I moved to new location, which was close to my previous address. During the first month, there was no internet connectivity issue. But from May 2020, Internet disconnects intermittently. Sometimes it disconnects 10 times per day, 1 minute each time. Sometimes 10 times per hour in the worst case, each time is less than 2 minutes. Sometimes it doesn't disconnect for whole day. This situation is frustrating because we are all working and studying from home. I tried 2 modems, 2 routers and couple of Internet cables. Nothing helps. I also called the Xfinity customer service. I was told the Internet signal was good. Sometimes I saw the error message saying DHCP error from ISP. Is it possible that the DHCP server may have any issues? How can I troubleshoot?