When all else fails, a search landing on Reddit is helpful. I found someone describing the exact scenario I am trying to solve - the modem passthrough router setup using AT&T's modem and Netgear router. And wondering if they should use the USB on the AT&T modem instead of the more elaborate setup replacing the router.

Before that search I found manuals (not very helpful) for the AT&T modem and it shows the USB port with but no information. Reddit's user had confirmation from several folks that while the USB is there, it doesn't have a current use so AT&T has it disabled. It might be used for charging (if you didn't have a gazillion other ways around the house for charging things.)

My goal over there is to find information, if I make a remark be sure it is helpful, and never get a down-vote. So far so good.