AT&T Wireless Internet

Okay I’ve got AT&T wireless internet (Guess ya say basically mobile hotspot). It’s from reseller company locally I guess you’d say. Well they use their own modems and what not instead of AT&T hotspot boxes or MiFi boxes. Well on Xbox One I have a strict nat type! They tell me that this is due to restrictions with AT&T towers not my modem/router they supply! But then they wanna tell me it’s due to contract or agreement they have with AT&T and or Verizon blah blah. So my question is does the towers have nat restrictions or is it hardware (modem/routers) that would be causing this?! I do not have admin access to modem/router they supply because they control it which I do not like but limited to internet providers in the area. 

So im tryin to figure out if they are Lying to  me and telling me that it’s tower restriction or if it’s acruallh their hardware they are using and just don’t wanna have to worry with having to set up port forward and all that stuff. I know if I use a Verizon MiFi box I can go in open nat type by port forwarding.