WriteAndFight,
Are you saying that you can find the session with your client but cannot join? I have this working on a couple of blueprint projects so maybe I can help if I can understand your problem exactly.
First things I would say though is
you do have these lines in defaultengine.ini:-
[/Script/Engine.GameEngine]
+NetDriverDefinitions=(DefName=“GameNetDriver”,DriverClassName=“OnlineSubsystemSteam.SteamNetDriver”,DriverClassNameFallback=“OnlineSubsystemUtils.IpNetDriver”)
[OnlineSubsystem]
DefaultPlatformService=Steam
[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=480
[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName=“OnlineSubsystemSteam.SteamNetConnection”
And, you do have the steam online subsystem plugin enabled?
When you find a session what do you do with the session information that you have found?
do you print to screen messages to let you know what is happening?
If you post your host, find and join blueprints it might help.