Make sure your SIM is SA 5G compatible

If you purchased a T-Mobile 5G compatible device, it came with a 5G SA compatible SIM card in the box! You just need to activate it.
If you're not sure you have the right SIM in the device, pull out the SIM card and see what's written on it:
R15: If you see R15 in pink on the SIM card, you're all set!