I believe that the vehicles still use cell towers as compared to a satellite link. The difference will be a stronger signal from the vehicle as compared to the cell phone (you can boost the cell phone signal with a booster or outside antenna). A car can use a MiFi but still you will need connect to a cell tower. You also will need to determine if you are looking for a mobile signal booster (while driving) or a stationary signal booster(like a camp site). The feds have set different limitations for each. If you are going on a road trip that you know will be challenging accessing a cell tower than you might consider renting a sat phone (see link above). Rental costs will depend on what you ultimately are trying to achieve.
The vehicle hot spot uses the ATT cellular network via a subscription you sign up for, so if there is no ATT cell coverage, the hot spot function isn't going to work beyond "inside the vehicle".