SIM card not registered Spitz GL-X750

First you should check if apn is correct. If not correct the APN.

Then, it is suggested to use ttyUSB3 as device to connect.

Check the screenshot in this post North American Carriers, ATT, Verizon, Cricket, T-Mobile Setup and Update 2021

If you have filled the correct info and still SIM card not registered, do a modem reset from the UI.