You will need to talk to your service provider. They are responsible for the SIM card functions.
Sim Card Lock
I have that option in Biometrics and security / Other security settings. I've never turned it on and it's default setting has always been off. The default pin is 1234 set by T-mobile. If it was turned on and set up with a custom pin number you'll probably need to enter that in order to turn it back off. You only get 3 tries with the pin.
If you're at all uncomfortable with trying to turn sim card lock off please follow @SamsungRei advice and ask T-mobile tech support for help.
Edit: below are images displaying the two separate unlock input screens after restarting the phone. The first one is the sim lock pin and the second is the system default pin unlock.
Galaxy S9 SM-G960U
Security Patch Level
December 1st , 2019