I went back to the T-Mobile store and got their modem/router, made sure it connected with my SIM, then moved the SIM over to the GL-X3000, and emulated the T-Mobile modem/router's IMEI. Once I did this, the GL-X3000 properly connected.
I wish there was a way for a Data only SIM to work without having to emulate the IMEI of another device, but that maybe a carrier issue rather than a hardware issue.