Local multiplayer works but not over internet despite being connect to same lobby

Hi,
I just finished this tutorial How To Use Sessions In C++ - UE4: Guidebook
For local tests, I launch the game twice, one creates the session and the other finds it, this works; both players are loaded into the map
When my friend tries from his pc (using Steam subsystem instead of NULL’s), the 2nd player can find the 1st player’s session, join it, but it fails to load the map
When I check in the dev portal for active lobbies, I can see that our lobby does have 2 players but only the one of us that creates the session is actually loaded in to the destination map, the other just has the menu map reloaded

Any links or ideas would be appreciated, thanks

Found out why, it was a typo in my defaultEngine.ini
Make sure
bIsUsingP2PSockets=true
is set correctly