Blueprint multiplayer not working via internet.

Every separated match needs its own Dedicated Server, as Servers in UE4 can only hold one World afaik.

If you have them connect to a Server to make changes and then have the same players play a match, you can simply call “ServerTravel NewMap”
on that first DedicatedServer. No need to switch the whole Server if you can switch the level.

A second pair of players still need their own Server though.