Do you have any bars? From your phone do you have your Network Selection set to Automatic? If that isn't working, try to manually connect to T-Mobile. Also try setting your Preferred Network to 4G rather than 5G.
Where are you in the US? Did you purchase a US roaming add-on? If so, which one? Does it include calls, texts, data? All of the above? It needs to say Roaming; international long distance and texting is not the same as roaming. Or do you have a Can/US plan? Is your plan at home active?
What phone are you using?