AT&T Smart Wi-Fi Extender (AirTies 4920) keeps disconnecting

I had my AT&T Smart Wi-Fi Extender - AirTies 4920 setup by an AT&T technician when our internet was installed. Our house is wired for Ethernet, with the hub plugged into the WiFi gateway. 

The extender is plugged into the Ethernet plug on the other side of the house, and I have paired it several times. It shows up in my Smart Home Manager devices for about 10 minutes every time and then disconnects. I have rebooted the extender, the gateway and the whole-house Ethernet hub several times, but the problem persists. 

I’ve not found a support article that addresses this specific problem that I’m experiencing. 

Thanks in advance.