There is no change. I conclude it must be an issue with the in-wall wiring and/or connector.

Sure thing. Here's the concept. Internet > AT&T 5268 ac wireless gateway (with switch)> TP Link Dumb Switch > GoCoax device1 > RG6 cable > GoCoax device2 > TP Link Dumb Switch > connected devices Like I said, as long as I don't put a RG6 connector on the RG6 cable into GoCoax device1, everything works as expected. Should the GoCoax device be plugged directly into the switch on the 5268 ac?