There's a PDF manual in the dedicated server directory. It has -crossplay listed as a command in the .BAT file you use to start the server.
Per that entry:
" -crossplay : Runs the Server on the Crossplay backend (PlayFab), which lets users from any platform join. If you do not include this argument, the Steam backend is used, which means only Steam users can see and join the Server."
If I recall correctly this is what enabled players to join my server via IP over a LAN like we had done before the Mistlands update.
Hope that helps.