What's the NB30 or MM-TD in an iPhone description in GSX?
The iPhone 6S exists in basically two versions:
Models A1687 and A1688 has support for LTE bands 1-5, 7-8, 12-13, 17-20 and 25-29
Models A1633 and A1634 has support for LTE bands 1-5, 7-8, 12-13, 17-20 and 26-30.
The extra band 30 is a special AT&T band. The iPhone supporting that band were sold by certain dealers in the US.
The NB30 on your iPhone description just means that you iPhone does not support LTE band 30.