@toztoz
If you can't get it to work this means the host either did something wrong (like not being in-game while the others join or bad port forward etc.) or has something still blocking the connection (firewall, antivirus firewall etc.). Make sure to allow Plutonium through your firewall (some antivirus have firewalls too!)
As a last resort, if you really can't get it to work you could try using a software like Hamachi or RadminVPN but you won't receive help for that! (make sure to switch to LAN mode instead of clicking on Play in the launcher)