Netgear C7100V Docsis 3.0

I have a Netgear AC1900 Wifi Cable Modem Router for Xfinity Internet and Voice (model C7100V w Docsis 3.0. ) Recently I upgraded my package to 600 Mbps internet but when hardwired with an ethernet CAT5 cable to my modem I am only getting 400 Mbps. After multiple trouble shooting calls with Netgear they said this modem looks fine and supports the 600 package and the issue is with Comcast. A Comcast tech visited and said everything is working fine to the house although they showed that only had a 400 speed service (according to the information he had which he showed me on his phone). Another call today to Comncast they said the tech had the wrong info and that yes I am paying for 600 Mbps! - and also they are telling me that Docsis 3.0 modem only supports up to 500 Mbps and I need the Docsis 3.1 to acheive the 600 Mbps. The tech specs on the Netgear website and the router packaging show this modem (with Docsis 3.0) supporting up to 1 gig. Someone is incorrect. Anyone out there have kowledge or solutions? Thanks you much.