Less than half the speed on Gigabit Extra with Ethernet

When I had just the Gigabit plan, I used to regularly  clock 900+Mbps on my desktop connected with ethernet cable. The configuration is Netgear CM1100 modem ==> Orbi RBR50 router==> Netgear 1 gig switch ==> Desktop. When my plan was automatically upgraded to gigabit extra, I am no longer able to get to the 900 Mbps speed. I clock 500 Mbps or below in the Xfinity speed tests. I totally understand that I can't get the full 1.2 GBPs with my current equipment, but why is my speed limited to 500 Mbps, suddenly?

I read somewhare that the CM1100 is only capable of 949 Mbps, but I'd be so glad to get that instead of 500 Mbps I am getting now! I also read that somehow reprovisioning the modem helped resolve the speed issue. How do I do that?