C6300 Frequent short internet drops

I have a C6300 modem/router with Xfiinity cable internet. I have used this exact set-up for several years without any issues, but over the last few weeks my internet has been pretty bad. Around every 20 minutes, I lose connection for around 30 secs. The outages are somewhat irregular, sometimes I don't have any for several hours and sometimes I have several within 10 minutes. It happens both through wifi and ethernet, and to all connected devices at the same time. The wifi network is still there and I remain connected to that, but I cannot reach the router control panel by entering its ip in a browser (the page won't load, similar to other websites). When I'm in a video call, it often seems like I lose uplink but not downlink, i.e., I can see and hear other people but they say that I am frozen.

 

I complained to Xfinity and they came out to measure the signal strength etc. and said all of that looked good. I tried modem/router reboot, factory reset, renew ip lease (which gave me the same ip back), changing wifi channel etc. but none of it seems to make a difference.

Any ideas?