Checking the host's global IP
To do this, the host should go to a website like WhatsMyIP, check their IP address and send it to the clients who wish to connect to their game.
Crash after clicking "Campaign Start"
This issue occurs if the game is installed in a folder path whose name includes a dot, for instance C:\Program Files (x86)\GOG.com\Divinity Dragon Commander\. To get around this problem, install the game into a folder path without any dots, like the default "GOG Games" folder.
Future releases of the game's installer will be set up to block dots.