Directly attached to a WWAN Router is a 4G data modem, 800/1900MHz in this generic illustration. The signal from the antenna passes through a dual-band amplifier before getting to the router. From the 4G LTE or 5G NR router anywhere in the vehicle, one should be able to receive sufficient signal propagated.

Another option involves the use of appropriate multiband antennas.

See amplifier options here.

Learn more on techniques on how to improve cellular signal.