Why doesn’t my T-Mobile SIM card work after unlocking my iPhone 10?

See—>How to unlock your iPhone for use with a different carrier - Apple Support

If you have iOS 14, note this:

If you see this message on your device with iOS 14, use these steps:

1. Go to Settings > General, and tap About.
2. Under Carrier Lock, you should see a message that says "No SIM restrictions." If you don't see that message, contact your carrier.

Did you restore the device with iTunes after getting information from AT&T? If you are still getting the SIM warning when you try to use your T-Mobile SIM, then the iPhone has not been completely unlocked.

The email message that you received from AT&T should have explained the rest of the process which would indicate a restore to factory. That process requires a computer with the latest version of iTunes. As I indicated earlier, that error indicates that the iPhone is not unlocked yet. Contact AT&T for assistance.

That message means that your phone is not unlocked. Contact the carrier again to get it unlocked.