T-Mobile has released a small software update for the Samsung Galaxy S9, S9 Plus and Galaxy Note8 on their network. The updates are mostly focused on bug fixes and performance improvements. The builds that you are looking for are as follows:
- G960USQU2ARF7 for the Galaxy S9
- G965USQU2ARF7 for the Galaxy S9 Plus
- N950USQU4CRF3 for the Galaxy Note8
All of these updates have been released by T-Mobile at this point so you likely can go to check for the update in Settings on your particular device.