How to make a simple network server?

Hey i am trying take a simple multiplayer game. I feel like im close but i am stuck. I have a basic network that i has a host and a client. To test this server, i simply make a build and run it as the same time as the game in unity. The 2 games successfully connect. However, when i test this server from computer to computer it does not connect. Why is this? And how do i fix this?

I have found that multiplayer problems like this can have a number of effects, but If I had to guess I would say to make sure your ports are properly forwarded, and that your game has registered with the unity multiplayer master server (unless you are hosting your own in which case best of luck.)

This is how I learned, it has good advice and straight forward tutorials for free: