I received a notice from Xfinity recommending I replace my modem because it is not capable of delivering the 200 Mbps speeds that are part of my plan.

I purchased an Arris SB6183. The modem and PC are connected to the router. After setting up the new modem and doing several speed tests I found I was getting the same download speed (83 Mbps) I was getting with my old modem. I contacted Xfinity. They ran some tests and had me connect my PC directly to the modem. When connected directly to the modem I was able to get 200+ Mbps download speeds. When connected to the router, I did not.
     
My router is a TP-Link AC1750 router that should be able to handle those speeds and higher (450 Mbps for 2.4GHz and 1300Mbps for 5GHz). After exchanging several emails with customer support at TP-Link, running tests, and taking screenshots they determined the router is not functioning properly, but since it is out of warranty they cannot offer me a replacement.

Should I get a new router?

According to this review there is a new version of the TP-Link AC1750 that has been improved by firmware upgrades. 

The model I have provides good coverage upstairs and downstairs.

or

Should I could keep my current router, return the SB6183 and get an SB8200 that has more than one ethernet port and connect the computer directly to the modem instead of to the router? If the modem is connected to the router, and the PC is connected to the modem instead of the router, is the router still providing the same level of security as if the PC were connected to the router?