[bigfatkitten]

Disconnect the LTE antennas, and terminate the antenna ports with 50 ohm loads. As far as the car is concerned, it's outside cell coverage.

If the car can't work without cellular coverage, then you've made a very poor life decision in buying it.

[fckgw]

This. Most vehicles have some fuse controlling the telematics or cellular modem. You can just disconnect that hardware from your vehicle.

[bigfatkitten]

On modern Fords you can pull the TCU fuse without issues. On other makes, doing that will throw errors.