Specifically, two 5G phones T-Mobile is selling today officially support two 5G bands, 600 MHz (n71) and Sprint's 2.5 GHz (n41). 28 GHz mmWave band (n257/n260/n261) is conspicuously absent and carrier aggregation seems to be limited to single 5G band and 4G LTE bands only.
All 2018 and newer iPhones support T-Mobile’s band 71 @ 600Mhz
https://www.macrumors.com/2018/09/12/2018-iphones-tmobile-600mhz-spectrum/