How do I connect a Netgear Nighthawk wifi range extender to the T-Mobile internet gateway

I have a Netgear Nighthawk EX7500 and can’t get it to recognize the new internet gateway. Neither Netgear (3hours on the phone over 2 calls) or T-Mobile(2hours on the phone). The router/modem light on the extender doesn’t light up so I can’t do anything with it. Netgear wanted $149.95 to diagnose on the 1st phone call and $75 on the 2nd call….no friggin way...highway robbery!!!