Probably not clear but Passthrough is what I need to ensure the TP-Link x60 router has a connection to the fiber network. My understanding it that the idea is to disable all unnecessary functions on the BGW320 and make it into a very simple modem. That entails disabling any routing function. The tech walked me through this which included what you see here: How to enable IP Passthrough on ATT BGW320-505 - Devon Stephens
The the primary x60 (wired to the BGW320) takes on full responsibility for routing.
I did happen to see the NAT/Gaming item, but I was baffled by the dropdown list. Do I choose the x60 and then create a Custom Service? Sorry for my confusion. Maybe this will walk me through the process