Running on Windows 10
Crucially, the registry must point to the correct CD drive.
The relevant registry entry CDROM_DIR can be found at:
Running under DxWnd may fix some issues.
To play online, ALL players need to forward ports 47624 (TCP) and 2300-2400 (TCP and UDP). Ensure there is no firewall blocking these ports, too.
Old games like this can be really hit and miss when it comes to network play. Sometimes changing the host can help. I have also had some success with Tunngle (no longer maintained) and Hamachi.
Unlock All Levels
To unlock all single-player scenarios in Anno 1602: Gold Edition, open Game.dat in a text editor and change:
Volume: -1485, -727, -595
Volume: -1485, -727, -580
To unlock all scenarios within the additional campaigns, add these Kampagne lines after Tutornr:
Tutornr: 0 Kampagne: 0, 3 Kampagne: 1, 3 Kampagne: 2, 3 Kampagne: 3, 3 Kampagne: 4, 3 Kampagne: 5, 3