Arris SB8200 Drops DOCSIS 3.1

I have been running my own Arris SB8200 for a couple of years now, with no issues. Recently, I noticed that after I restart the modem, the download starts off in DOCSIS 3.1 (blue light), but switches back to DOCSIS 3.0 (green light) after a few minutes. After a couple of calls with Xfinity tech, who were very nice, I was told there's nothing they can about it, because they can not diagnose my modem, since it's not Xfinity. That's a bit weak if you ask me! I have tried to restart the modem, reset to factory settings, and the same. Blue for a few minutes, then back to green. To add insult to injury, I received an email saying that my download speed was upped to 400mbps from 300mbps and guess what, I used to get over 300 before this "upgrade" and now I'm lucky if I hit 200mbps. All being equal, nothing has changed. Same modem, same Airport Extreme router, same cables and connections. Any help would be appreciated.