I also have kinda same problem. I play 1vs1 games with no problem. But when comes to custom games with more players game crashes.
1.
I join the lobby it connects to all players and then it starts to ask to reconnect /connection to host timed out/ to host of lobby, i see all other players usualy most of them and can chat but cant connect to host and after couple of minutes games crashes.
2.
or second i cant join the lobby. FAF is running with no problem but game gets to lobby and freezes. I look at lobby frozen screen for 1min or 2 and then it crashes.
Firstly i opened all ports on my router but that didnt help, then I tried UPnP (enabled it in FAF and on router ).
I pass test proxy.