If you have a T-Mobile version of the OnePlus 8, you can go to your phone’s settings, and check the latest T-Mobile software update is available for your device.
T-Mobile is sensing OTA software updates to the OnePlus 8 devices on its network. This new update includes 5G support with two new bands. You won’t be using these bands right away, but at least your device will be ready when they become available. The update includes support for 5G bands 2 and 66, in addition to the bands 5, 41, and 71 that were supported at launch.
T-Mobile’s 5G network works on the 600MHz band 71, it’s working to support more bands in the near future, which would also include the just added band 2 which is the 1900MHz and band 66 which is 1700/2100Mhz.
You can go to the device’s Settings > System > System Update > Check for update, and see if this software is already available for your device.
Source Android Police