My squad, containing mortar crew , is going to attack an opponent's squad which is in range, but doesn't has LOS. So I'm using a spotter, which has LOS, but is far away from the target. Is there any limitation about how far away the spotter might be from the mortar's target, as long as he has LOS? Any distance? Spotter's long range distance? Spotter's normal range distance?

There is no limit on the range of LOS so as long as the spotter has LOS the attack can take place