Did they just ask to reboot the gateway and wait for a callback? I understand they obviously don't have the staffing to be able to answer every phone call, and I agree that it sometimes can resolve issues, but when it doesn't, it's very difficult to get help, and they usually don't see anything on their end.
Do you know how to login to your router? Maybe you can check the and post the downstream and upstream channel bonding values. If you login to the gateway at 10.0.0.1, there's a Connection > Xfnity Connection page where you can see these values. SNR is the signal to noise ratio, and should be between 35-45 I believe. I also think dBvM power level should be anywhere from -7 to +7.
I know, and on Saturday, it was 4 hours. I get the same things, where they text me after the modem resets and they ask if it the issue is resolved, I reply no, and then it says there was an error scheduling a callback. My bet is, they aren't able to handle the load right now because everyone is working from home these days. It's NOT an acceptable answer, though, especially when I get emails from them stating they are doing all they can to make sure we have stabilty, etc..