maybe this post has the same problem upgraded from 5.2 -> 5.5.1 and i no longer can make steam p2p session work .
And there the Advanced Sessions plugin was the solution. I am using the binaries from here Advanced Sessions Binaries – VR Expansion Plugin . Maybe the source here GitHub - mordentral/AdvancedSessionsPlugin: Advanced Sessions Plugin for UE4 is still newer and fixes the problem? I guess I will wait for new binaries 