Freezing when in same network.

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I just started training two new people, brothers, which want to play from the same network.
Although the game always freezes predictably after some time, because they are behind in packages. No connection window pops nor a desync.

Is it impossible to be in the same LAN and play over FAF?
I can play with one of them if the other isn't in.

What we have tried:
1) Nothing special at all
2) Forward UDP for one of them and let the other use the Proxy
3) 2 but in reverse.
4) just the two without me -> same problem, so its just them.