LB2120 Fails to Connect to Wired Internet Service

I purchased the LB2120 to use as a backup for my wired internet service.

I've connected the LB2120 in the Bridge Mode between my Motorola DOCSIS 3.1 Cable Modem and Night Hawk R7800 router. The Cable Modem is connected to the LB2120 WAN port and the R7800 is connected to the LB2120 LAN port. When I select "Connect" to internet with Failover in Auto, the LB2120 will only connect to the internet via a cellular signal. If I select "wired broadband only", the LB2120 will not connect to the internet at all.

I've spent several hours on the phone to Netgear Support troubleshooting this issue. They gave up and decided the LB2120 must be defective. I returned the  LB2120. I received another and it behaves exactly the same. I can't find any solutions in the Community or surfing the web. Can anyone help me get the LB2120 working as its advertized, as a backup to my wired internet service?