Sim Card Not Registered is indicative of a no cellular signal message, and not a message indicating we have not activated your sim card. Rest assured, your sim card is fully active, and service will begin working as soon as you're in a stronger coverage area.

You may verify this by clicking the AT Command link at the bottom of the T-Mobile/3G/4G Modem box area, then from the shortcut drop-down, select the Request Network Information option and then press the send button. You will now see a 'No Service' message.
If you do not see a 'No Service' message, but instead see a message with the letters 'WCDMA', then your device is connected to 3G signal, because the 4G or 5G signal in the area is too weak. Try a new location to get connected.
Powercycle your device, and try set up again.
If you have low signal, please try using a different antenna if your device supports them - options available here: High Gain Antenna Options