So as I understand a client can connect to server it them both use same subsystem.
But dedivated server starts with regular engine networking session.
How to force dedicated server to use online subsystem to start a session?
Ok It opens listening socket conection but client does not see it. Turns out that you have to actually create session when server starts “again”.
What i did - i made a default server map that start a session and travels to another gameplay map. Than client can see that server.