Problem in Multiplayer - Friend can't select a plane

[EPICAC]

So I've been enjoying Cliffs of Dover for a long time, knew there was multiplayer, and convinced a friend to buy it so we could play together. We tried to do so recently and ran into problems.

Once I got a server up (he had to connect directly to my IP and port as opposed to just finding it in the list) I picked one of the stock missions and selected start battle. When my friend joined in he could see the list of planes but couldn't fly any of them because they all said WPF.unavailable.

Looking around I see this to mean that he can't spawn into a plane more than 20 km away. But that doesn't make sense because there were only six spitfires, all flying in formation, well closer than 20 km.

What are we doing wrong? We just want to fly missions together. Why is this so difficult?