Why can't my FO order his mortars to fire

[Mobhack]

It may help if the F0 (as well as the mortar team) have radios. Did you check that?.

And if they do have radios, then they also need to be in contact (which is not guaranteed).

Next - the mortar needs to be a proper indirect fire capable mortar element, and not a direct-fire only type (like some 50mm and other "commando" ones are).

Your AO is an artillery observer - better at it than a company or platoon commander, but not as good as a proper FOO. If he cannot see the target - then fire the mission unobserved, if needed. Less accurate than observed fire, but often good enough.

Andy