Paying for 2.1gbps, getting 700mbps

I've tried everything. I had a Netgear CAX80, which is on Xfinity's website as as supported device, and it only gave me 500mbps.

I get a technician out and he connects my coaxial directly to his device, and even put in a splitter as a test. His device was reporting 1.5-2.0gbps. He suggested it was the router.

Ok. So I go get a Surfboard S33 (also on xfinity's website that it will support over 2gbps), and a TP link router. Both have 2.5gbps ports, and a CAT-6 is connected to the 2.5gbe port on the modem and the 2.5gbe port on the router.

After activating it, and plugging my computer in, after spending nearly $400....a 100mbps increase.

This is unacceptable. Considering I'm using equipment that Xfinity supports, as well has having a technician confirm my connection is alright, it's honestly highway robbery.

I need to know what's going on with the internet that makes sense.