Xfinity support

I’ve had Xfinity for quite some time and noticed I was not getting the download and upload speeds I used to. I had a tech come over and check the coax line from the box and my Netgear CM1000 modem and determined that the issue was the modem because my download speeds were 500 and upload was 5. I purchased a brand new $500 Netgear CAX80, top of the line modem/router, and found the issue persisted. The Xfinity assistant is telling me that my modem cannot keep up with the speeds of the internet (which is simply not true.) Is there something that needs to be done on Xfinity’s end to let my modem receive the internet speeds it’s being sent?