GL7500 and Verizon

Has anyone had any success connecting the GL7500 to a Verizon network. I immediately get a registration denied (+CREG: 3) when I power up the modem. I’ve been in touch with Verizon support who told me it may be a modem problem. They could see the modem’s sim but could not offer an explanation why is was getting denied.
I’ve tried two separate GL7500 (both with Verizon fw) and two sims with multiple account configurations. I’m using a Taoglas MA750 antenna.