Bummer man, lol. So I gather it highly dependent on the carrier? For instance it may or may not work unless I can get AT&T to enable it on their end?
That is part of it. The other side of the coin is that the phone is capable, but we lack the capability to enable it ourselves. For the 950 XL and other Lumias, we have the ability to provision modem settings through an NVI update or modem configuration reset. Just like how it is possible to remove or add lockable LTE bands on many Lumia phones by using either nvi files or modemconfig cab files.
I do not know enough about the Elite X3s modem/cellular settings in how they get provisioned or set to be able enable it. We know the registry settings enable it in the UI but that is as far as we get with the X3, for now.