From what I heard, the ROG Phone 5 does not feature 5G mmWave probably because countries outside of the United States don't feature it.
I think it would be great for ASUS to work with carriers in the United States such as T-Mobile, Verizon, and AT&T to try to get the ROG Phone 5 to work with as many network bands possible in North America. Just like VoLTE, the lack of full 5G compatibility are basic phone features that ASUS is overlooking once again.
Phones usually have different regional variants to better fit the different network types in different countries (e.g. GSM and/or CDMA compatibility) so I'm hoping that the North American version of the ROG Phone 5 will also be compatible with mmWave. Otherwise, it would seem like the ROG Phone 5 will become obsolete very fast.