Borderlands, not able to host (GameRanger)

Me and some friends recently downloaded Borderlands (via The App Store). The game's singleplayer part is working perfectly, however we kinda bought the game to play it co-op.
All of us downloaded the gameranger client to host our own lobbies. However None of us where able to do so (I was able to host a game twice, but after that we all expierenced the same problem).

Here is a simple explanation of what happens:

1. We open GameRanger
2. One of us starts to host a lobby
3. The rest of us joins the lobby
4. The host clicks start
5. The game boots
6. We get to the main menu (the same one you get when playing singleplayer)
7. There a load screen, and thats where it stops, it just keeps on loading and loading, it doesn't go past the loading screen.

we are forced to force quite the game and to force quite GameRanger (it won't respond).

We tried to look online, which seems to tell us to forward ports, however none of us seem to have a clue on how to do this + the once that do know something on this topic don't seem to find where to do so?

Thanks in advance,

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