I've searched a bit, and no solution found, as the answer I found from a mod was "It's working for me.".
I use ARKON, I insert the IP, the Port and Password found in the Dashboard, and the only thing that is happening is, Connecting...
And yes, RCON is enabled in GameUserSettings.ini , could this be caused by the server running in Whitelist mode?
*EDIT* So, I tried another RCON tool:
And this worked.