I just tested it.
You are correct. Using, Ural375, ATMZ-5, CP SKP-11, nothing happens, not even a reply.
When I add the FARP static objects, Command post, ammo storage, fuel depot, it does work.
So I tested it a 2nd time with just the trucks ( Ural375, ATMZ-5,) and the staic object (Command post) and it worked fine.
So at least you can work around it for your mission, but It should be checked out if the CP SKP-11 is meant to act as the command post.