Whoever is running the server should open it first, then the other person should open their client (and the person hosting the server if they want to join as well) and join using the IP (you will need their public IP address in order to join, make sure they have port forwarded otherwise you wont be able to join).
The server should be displaying a "Navmesh generated (server is ready)" message before anyone tries to connect to the server.
I know its a little bit confusing lol, but let me know if u have anymore trouble