I started something like this a while ago its not a full system its just the foundation. You could expand on it though. All it does right now is when you click “Host” it will host a game with your Ip. But when you click join it will ask you to enter an ip. Then it will take the text you typed and send it to an append which will be added to a console command to join the game of that ip. Like i said very basic but a good stepping stone :}. But Hey! It Works!.
1: Make a Widget Blueprint and name it Multiplayer.
2: Make a Host and Join Button inside that widget blueprint.
3: Rename both buttons and make a bind for both.
4: Make another new widget blueprint and name it Join.
5: Inside the Join Widget Blueprint make a Editable Text and a button.
6: Rename the editable text and at the top right make it a variable.
7: Set a bind for the button under the editable text.
8: Open the graph in the Join widget blueprint and copy the picture below.
Join Widget Blueprint: (The “Ip” variable is the editable textbox") Make sure you add a space after open in the append.
9: go back to the Multiplayer widget blueprint and open the graph.
10: on the Host button follow below.
Host Button:
11: on the Join Button.
And that should be all you need to do. It will not work in the editor you will need to Package your project and open two versions of it on your desktop. Let me know if you have any further trouble.