If you have a T-Mobile version of OnePlus 8, you can check your phone's settings to see if the latest T-Mobile software update is available for your device.
T-Mobile detects OTA software updates for the OnePlus 8 devices in its network. This new update includes 5G support with two new bands. You won't use these tapes right away, but at least your device will be ready as soon as they're available. The update includes support for 5G bands 2 and 66 in addition to bands 5, 41 and 71 that were supported at launch.
T-Mobile's 5G network is working on the 600 MHz band 71 to support additional bands in the near future, including band 2 (1900 MHz) and the band just added 66 (1700/2100 MHz).
You can go to Settings> System> System Update> Search for Update and check if this software is already available for your device.
Source Android Police