Hey Nox_A15,
Null also seems to be working according to the project I’ve set up for testing above. Feel free to download it and take a look at it.
C to create a session and J to join a session.
Did you ensure to add:
[OnlineSubsystem]
DefaultPlatformService=NULL
to your DefaultEngine.ini?
Also have you ensured to restart the editor if it was open when you added this to the .ini?