So I am able to create and join a session with your project no problem:
The next step for you is to figure out why your connection is blocked. These are the ports required for listen servers:
-URL ports
-Port=7777
-PeerPort=7778
-OnlineSubsystemSteam Port
-GameServerQueryPort=27015
-OnlineSubsystemUtils.onlineBeaconHost port
-ListenPort=15000
You can try to open these with your router / modem, to see if that fixes the issue.
You are the first person that I have come across who has this issue. If you continue to have issues with people not being able to join, you may want to call your ISP and see if they block ports from being used.