I'm in a suburb of Cincinnati, for which T-mo supposedly just rolled out the 600 mhz (which should be band 71). My unlocked S9+ still shows it is pulling signal from band 2 this morning when I installed an LTE analyzer app. This is the same band showing when I first bought the phone about 4 weeks ago.
According to this FAQ on the Samsung website, the unlocked phone does have band 71. I asked on another thread if it is possible that access to the band still somehow is controlled by T-mo via software (and therefore unlocked phones may not get it), but didn't get any helpful answers.