It seems like Samsung has made some changes to the international Exynos Galaxy S9/S9+ and not all T-Mobile 4G LTE bands are supported this year unlike previous Galaxy phones. AT&T is not affected but if you are on T-Mobile, just know you may get stuck on HSPA+ most of the time if you buy the international Exynos Galaxy S9/S9+ models.
Update: The issue has been fully fixed after doing a software update, it was an early bug that is fixed easily with software update. 4G LTE works great now and picking up 4G+ signals also, please disregard this post thanks.
Affected models:
SM-G960F, SM-G960FD, SM-G960F/DS
SM-G965F, SM-G965FD, SM-G965F/DS
A good alternative is any previous Galaxy international models like the Galaxy Note 8, S8, S8 Plus, S7, and S7 Edge, they all support T-Mobile 4G bands fine.