To be clear, you should not use a power adapter that is outside of the power requirements of the 4G LTE router.
That is just a bad idea and will void your warranty for using the device out of specs. But if you are using a car adapter and then plugging into the normal wall adapter in a daisy chain scenario then you should be fine. That way the wall adapter can regulate the power going into the 4G LTE router and keep it working as expected. If you have problems the first troubleshooting step should be to replace or swap out the car adapter.
A word of caution: Cars are poor locations to maintain mobile broadband equipment. You have to be careful to keep the car at a reasonable temperature and avoid direct sunlight. Many folks in the southern US have discovered that a VZW device left on the seat of the car for a few hours will leave them with a hot lump of plastic and silicone.