Setup Dedicated server (installed by steamcmd preferably). Then install another copy of the game via steam CLIENT.
Issue with authentication through scmd explained again here: http://feedback.arma3.com/view.php?id=13857
Then attempt to connect, if the cleint 'HC' does connect confirm that EXTERNAL clients can connect to the DS and that the session is not "locked".
I found that setting localClient[]={81.19.212.50}; to localClient[]={127.0.0.1}; results in HC being unable to connect.