I have read that the only unlocked in the box iphone XR in the US right now is the t-mobile version which is the A1984 model. You should be able to buy it and put any carrier sim in and use it. HOWEVER it is not the same radio as the Europe version which is the model A2105, looking at specs it has LTE band 71 that the US version doesnt have. Dont have any idea what that would mean for you real world performance wise.
All US XRs are A1984, the modem is the same. And you're backwards, the US has band 71 while the EU one does not.