Doesn’t this have more to do with the hardware bands available in the device rather than the carrier software installed on the phone? Perhaps as part of the race to the bottom on price Xiaomi forgoes hardware components that allow a given phone to operate on bands that are common outside the region it was sold in or the carrier it partners with. It would make sense if that was the case, because the people spending $20 USD on a phone typically aren’t going to be traveling a lot anyway