I have an early x750 that has been having issues with the LTE LED illuminating consistently. It works as expected sometimes and other times it does not work at all. LTE is the only connection in use. Restarting has no effect. It seems to be a hardware issue, though physically manipulating the device to expose a cold solder joint has no effect.
This router was one of the ones that was recalled for having the extra capacitor installed; not sure if that is related. This router runs 24/7 and has done since it was purchased.