I have a recently purchased GL-X300BC4 that is running 19.07.8. At first I was able to connect to Tmobile but only inputting the APN. About a day later I started getting the error SIM card cannot be registered. Please check: 1. You are using the compatible modem for your carrier; 2. Whether a specific APN or data plan is required. Running AT+QPRTPARA=3 outputs error
I also tried using QCM as described here
I currently have two tmobile sims and a pepwave br1. Both sims will work in the pepwave using the same APN but both give the same error in the X300
edit: also of note the 4G LED is on