Suddenly slower speed with Arris SB8200

I have had an Arris SB8200 cable modem for several years and always got above the speed I paid for with Xfinity, currently 100 mbps. I happened to test the speed a week ago and it was 120 mbps, so all was fine. But today, I had some connectivity issues with a Zoom class and tested the speed. It was around 50 mbps, so I called Xfinity. The agent said she saw some issues, made some changes on her end, and had me test the speed again. It was unchanged. She said there had been a "mismatch" which she addressed and I now needed to call Arris. I did so, they said it was out of warranty and that I needed to use their online chat. I tried that and they directed me to a FAQ sheet which gave ranges for a variety of values. All downstream values were within the good range. However, the power levels for the upstream bonded channels were high at 52, 53, 53, and 54. I reported that in the chat, and the Arris agent told me they couldn't do anything about it and to call Xfinity. I expect when I call back they will send me back to Arris. Is there anything I can do about those levels? Will I have to purchase a new cable modem, just to have one in warranty?

Thanks for the help.

Robb