Hi,
I had the same problems.
But after reading a post that someone could host a game using vpn (but no one could join) , I had a moment of enlightenment that solved my problems.
I did the following:
1) Ethernet port disabled.
2) Connected via hotspot on cell phone.
3) started game, multiplayer suddenly worked!
4) quit game, disabled hotspot, enabled ethernet port again.
Suddenly it works again. no idea why.
But it doesn't hurt to try, does it?
Hope it helps you too
PS:
Guess that somewhere an accesslist is working, which does not let you into the game. However, as soon as you enter with another connection, you are entered there again.