How do i fix this Problem in Multiplayer Mode (F1 2018)

[jonathan1299 0]

The Problem is that I can‘t enter the Multiplayer Mode. I can See the lobbys but I can‘t join. After a Long loading Time it says „error, couldn‘t Join“
Please help me to find a Solution!

[IlluminatiCat 5]

Try opening all your ports (DMZ) and see if that improves the problem.