Nighthawk AC1750 requiring sign in

I have had my #Nighthawk AC1750 for over a year and never had any issues with it.  For that last year it has been connected to Comcast broadband internet with a 4GL LTE Failover modem provided by Comcast.  Recently I changed from Comcast Business to Xfinity residential internet and purchased a #Netgear LM1200 4G LTE modem to use as a failover.  The modem has a T-Mobile Sim card and works without issues when connected directly to my laptop via Ethernet. The problem arises when I try to use the modem to provide internet for the router.  While all of the lights show that there is active internet, the router does cannot connect to the internet.  The Ethernet connections simply show no internet and the Wifi network asks for a sign-in.  However, when I select "sign-in" I get taken to a website that is not available.  I tried contacted Netgear customer service but they insisted the problem was a configuration problem on the router which they could not help with unless I bought a service plan for the router.  I have updated the firmware on both products and rebooted both multiple times.  Has anyone else encounter this problem?  If so, were you able to solve it with configurations?  Did I just get a defective modem?