First, make sure the computer hosting the server have forwarded the port 1254 on router (you can specify the port on server creation if you wish).
Then inform the server ip to all players. They should go to "Connect to IP" and enter that ip.
You can find out your internet ip in https://www.whatismyip.com/ (the IPv4 is the one that matters)
When this happens it's because the host didn't have the right port open.
Search for "port forwarding" on google. Knowing the model of your router might help.
A quick solution is to enable DMZ for the hosting computer.
If everything fails or seems way too complicated you may use a LAN simulator program like Hamachi or Tunngle.
Just to be safe: if you have a firewall installed make sure it is not blocking the game.
