I have AT&T LTE WIFI in my 2017 truck and also LTE data / hotspot / etc.. on my iPhone. In the real world signal strength doesn't go up with the truck. Rarely if ever have I seen the truck get a signal when my iPhone itself could not. Yes, the truck has an external antenna BUT it is not boosted / powered to improve signal strength, it's simply an antenna.
Nonetheless there would be 2 reasons for you to get the service in vehicle:
1. If your truck has AT&T and your iPhone is Verizon - you can likely get coverage with one network you may have it with the other.
2. With the AT&T service for vehicle hotspots (at least with GM) your data is truly uncapped from a usage and speed perspective. If you use a lot of data this comes in handy. My AT&T account shows X Gigs used of 999 Gigs available each month and speed is never throttled after X gigs of data usage as with traditional line service
Good luck.