Join session Steam succeeds but doesn't really join

So I can create a session successfully using the online subsystem steam, and I can successfully join it, however it doesn’t actually join the game, as I cannot see the other player and the number of online players always stays 1 even after joining. Here is a link to the project, completely blank except for the necessary files.

https://drive.google.com/open?id=1tWHmNZEL87S6uwLUKkTWR8PWDCiQn2ON