The OnePlus 8 is available as an unlocked device, but T-Mobile and Verizon have their own versions. T-Mobile and OnePlus are rolling out an update today that enhances the phone's 5G support with two new bands. You might not run into those bands right now, but now you'll have them for the future.
The new update (build number 10.5.8.IN55CB) adds 5G bands 2 and 66. That's in addition to the bands 5, 41, and 71 that worked on the phone at launch. Band 2 is 1900MHz and band 66 is 1700/2100Mhz, sometimes known as extended AWS. Currently, T-Mobile's nationwide 5G network runs on the 600MHz band 71, but it's gearing up to support more in the near future.
This OTA should reach all phones in the next few days, but you can try to pull it down immediately in the system update menu. That's in Settings > System > System update > Check for update on OnePlus devices.