If you’d like to power the router inside your car/van/camper/RV, you’ll need a DC-DC converter.
This will transform 12V DC to 5V DC.
It should at least have enough power to deliver enough for the router and any USB (Modem or Tether) device.

V(olt) have to be the same
A(mpere) sums up.

5V/15W = 3A
5V/7,5W = 1,5A … And so on

You need either the Watt or Ampere to get the consumption.