Not sure what provider you would be using, as an example: 

Port forwarding is rather straight forward - I did try this, not with verizon but using LTE on a local provider... 

worked alright - I assume your remote sites are mtn. top locations, one thing to consider is most of the time the signal 

from cell carriers is directed down more then out/up - may not be the most reliable at extreme elevation... 

vnstat: - looks promising for measuring usage -

Mark

KC7DMF