Hi, I never got the online part to work correctly in game. So I found GameRanger, they take care of games the devs stopped supporting and so on.

Test this if you can't play the game online any other way.
This works for me and my friends.

1. Download GameRanger [www.gameranger.com]
2. Install GameRanger and Make a Account.
3. Start GamRange and make sure it fund Anno 1404: Venice
4. If not found goto Edit, Options and then scroll to Anno 1404: Venice. Now Browse and find ur exe, when done close options window.
5. Tell your friends to do all steps above, and add them to your friends list.
6. Up right you see Host press it and choose Anno and desired settings.
7. Wait till your friend is ready.
8. Press ok, allow firewall and continue NAT settings or what it say.
9. A new window will open wait until your friend/friends are in.
10. Press Start and the game starts up automatically and makes the most settings for you.
11. Your friends game should start automatically when yours are loaded.
12. If all worked you should see your friends and be able to start the fight :)

Hope this helped somone.
Good Luck

Freddy out!