In order to create a session and search it use the functions CreateSession (the one provided by Oculus, not the default one) and FindMatchmakingSession function.
Configure a matchmaking pool on your oculus dashboard (dashboard.oculus.com) and pass the same pool’s name to the functions CreateSession and FindMatchmakingSession.
What is causing problems now is the JoinSession method. Oculus does not provide a Blueprint function to join sessions, so when I use the Unreal’s one I get the same error message: “Cannot map local player to unique net ID”