Xiaomi had years to break into the US market before this "ban". It is not the fault of the telecoms or government for xiaomi's failure to enter the US. It has more to do with the US market and the way people buy phones. Most people buy phones through carrier subsidies or finance. Why buy a $150 xiaomi phone when you can get the latest iPhone or Samsung at $300?
Telecoms also sell phones that are popular here and people want to buy. Sony as big as they are tried to enter the US market and even partnered with a US telecom and failed miserable.
The US market is tough to get into because of how consumers think and buy here.
Oneplus (BBK electronics/Oppo) seem to be doing okay in the US though.
Xiaomi phones will be "connected" in the US but you would be bouncing between 3G and LTE depending on where you are. If you are outside of big cities coverage will be poor.