I do not have an external (real) IP now and can not test it again. But before recent patches I just:
- went to Steam folder and found launcher.exe file
- made a windows shortcut to desktop
- added -server only to this shortcut
- launched server via this shortcut
- launched client from Steam window
- connected from client to my server
Maybe some ports must be open.
I can not figure out how to use Lobby with dedicated server because now I am behind a router and my dedi is not starting the usual way. Can not test my mission efficiently