Go into server.cfg and manually add it. like so on the bottom.
# If you want players outside your LAN to be able to connect to your server
# and you are behind a router,
# you must also forward TCP port 2593 to your private IP address.
# The procedure for doing this varies by manufacturer but generally involves
# configuration of the router through your web browser.
@Address=78.178.69.251
# The Port to bind the Listener to.
# Default: 2593
@Port=2593
# If you want players outside your LAN to be able to connect to your server
# and you are behind a router,
# you must also forward TCP port 2593 to your private IP address.
# The procedure for doing this varies by manufacturer but generally involves
# configuration of the router through your web browser.
@Address=78.178.69.251
# The Port to bind the Listener to.
# Default: 2593
@Port=2593