UPSTREAM POWER LEVELS TOO LOW. NEED HELP PLEASE.

I am paying for Extreme Pro Plus 600Mbps. I'm getting between 67-81Mbps on one channel and around 350Mbps on the other. He told me my modem (SB6190 3.0) is not going to get over 400Mbps.  And that I need a 3.1. Ok. Why didn't they tell me that when I upgraded to this tier?

I then called my modem company Arris, and spent another 2 hours on a chat with them. They told me that this modem is capable up to 800 or something Mbps. But he also did several remote tests and told me to perform tests as well.

He determined that my Upstream Power level is too low - 38.75 to 39.50 between 4 channels. He told me to call Xfinity and tell them to raise the power level on the upstream. I've been texting now for 2 hours with Xfinity and they say all is fine. No problems.

WHAT DO I DO??? So frustrated. I don't know who to believe. Thanks.