Hey there @Qeraga! So in singleplayer games it’s totally fine to just use Open level, however in multiplayer, the dynamic changes a bit in that you now have a session, IP, etc. Even if the server opens a map, that just makes the level load on the server but it doesn’t transfer the clients. You actually need to use ServerTravel! It sounds like it’s used for actually changing servers (which it is) but also changing levels on a server.
I’ve brought both documentation and the network compendium writting by amazing community member Cedric Neukirchen. I used this as my multiplayer cheatsheet for so long I can’t ever not recommend it.