How to make AI Katyusha Trucks attack a zone or target?

I’ve spent a couple hours trying to get AI Katyusha Rocket trucks to fire upon a city below them in a valley. Is there any way I can get them to fire? I’ve tried the unitAttackTarget trigger and set their attack types and everything. Is it a problem of distance? Any help would be much appreciated.

The AI in ground vehicle can’t launch rockets (only ATGMs and IR missiles).