Connect via coaxial cable to the Ethernet port.
Connect via coaxial cable to the Ethernet port.
It's true that a gateway functions similarly to a modem, but as a gateway it brings extra factors like double NAT. I think there should be an easy way in the gateway's web settings to switch it to bridged or modem-only mode. I wonder if Xfinity restricts this option so a technician must handle it? That seems inconvenient for customers. The person asking needs to connect the coax to the gateway/modem, then link ethernet from there to the router's WAN port. After that, set the gateway to bridged mode—either by adjusting it yourself or contacting your ISP for assistance. Once done, follow the usual steps to configure the Netgear router and everything should work smoothly.
Starting bridge mode on the Xfinity gateway uses the standard login details given by Xfinity: username admin and password password. The Netgear router should have included a setup guide and cables or an online link for instructions. Did you forget to apply them?