your connection is not private problem

I recently signed up with Xfinity and have a new modem. Ever since Xfinitiy service started, I get "your connection is not private"  with a note "NET::ERR_CERT_COMMON_NAME_INVALID" on a number of websites. It further says "This server could not prove that it is ***.com/org/etc; its security certificate is from register.be.xfinity.com. This may be caused by a misconfiguration or an attacker intercepting your connection".When I click proceed to the particular site anyway, it takes me to modem activation page. When I go through modem activation again, it does not solve the problem. I can sometimes see those websites that seem to have been blocked on random occasions and I can certainly see those websites when I am elsewhere like a public wifi. I didn't have a problem with Spectrum. I initially thought this could be a chrome problem but it also happens with microsoft browser. I talked to a comcast tech and he says it is a modem issue. But I don't think it's a modem issue since I am connected to internet otherwise. Is there anyone out there who can help solve this problem?