I have the same problem basically. I tested a few things (P2P Adress, IP, etc.), all with and without ports.
What worked in the end was “Open steam.XXXX” where XXXX is the “OwningID”.
Im also at a point where my server is shown in the LAN Tab of the Steam Server Browser (although its not created as a LAN Server.)
My assumption for my own problem is now that I have to get my port forwarding correct.