Other than the port numbers, this issue and solution are not specific to JJ2 - online games with decentralized user-hosted servers, e.g. Minecraft, generally require the host to forward ports for clients to be able to connect. Unfortunately, increasingly many ISPs employ an approach called CGNAT, which prevents port forwarding from having an effect. If all you want is to play with a friend and you find that you have no luck with port forwarding, you may want to try software such as Hamachi or ZeroTier - just read up on them online and make sure you and your friend know what you're installing.