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 found Anno 1404: Venice by clicking Host button.
4. If not found goto Edit, Options and then scroll to Anno 1404: Venice. Now Browse and find ur Addon.exe. If u got Invalid Selection, then copy the Anno 1404 folder to your C drive, preferable C:\Games\Anno 1404. Now try browse Addon.exe agin. When done close options window.
5. Tell your friends to do all steps above, and add them to your friends list.
6. Now press Host button 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 :)

Info site I made with pictures how to host in GameRanger [freddy.zapto.org]

Hope this helped someone.
Good Luck

I saw very nice online guides here, I might try them when I got the time. Its been years since I last played online (not awailable to buy on steam then). But I got a couple of questions on how to play this Online, so I figured why not put this here for all to se. I tested it last week so I know it still works, and it was 4 servers up then. :)

Freddy out!