Use with Steam OnlineSubSystem

[azukaar]

Hello!
I am using Steam for multiplayer.
So when a client connects, after connection to the server it then connects to the second port too through TCP.
It works fine normally, but with Steam's network OSS it fails
I'm assuming that for obvious security reason, the server needs to tell Steam to accept connection on both port, but I have no clue how to actually allow the second connection?
Or is it automatic with FSocket and the answer is somewhere else?

PS : When I connect I correctly user steam.123123123 as the IP