“Details have been hard to come by on bands, but does the unlocked version on the S9 and S9+ support T-Mobiles new band 71? There have been conflicting reports of only the T-Mobile locked version having band 71 support vs. both T-Mobile and unlocked.”
Yes, it does. band 66 and 71 are supported. I went the Samsung USA website to get your answer. I also added a screenshot of the site network frequencies and data bands below. please protect your phone with a case that has a raised lip and shock resistance.
webpage - https://www.samsung.com/us/mobile/phones/galaxy-s/galaxy-s9plus-64gb--unlocked--sm-g965uzpaxaa/#specs