Will a Xiaomi Redmi Note 9 (international version) work reliably in the United States (comparable to a domestic phone)?
I was going to try and leave a comment instead but my reputation is not high enough.
A useful site that breaks down frequency bands is
https://www.frequencycheck.com/compatibility/Jxq7inG/xiaomi-redmi-note-9-global-dual-sim-td-lte-64gb-xiaomi-merlin/united-states
This will show you the bands of each network in the US and will state whether the phone supports it or not.
I can see that it is stating that only 2 of 5 LTE bands are supported on T Mobile for this phone. So it is likely you may experience some issues with 4g mobile signal depending on the bands available at the location you are at.