i had the same issue. The reason was that the join session function uses for network connections the IP6 format.
But due to my router settings it only worked with IP4 addresses. Some routers use ip4/ip6 conversion. It might be related to this, but not sure.
(example IP4 format:192.168.178.69 / IP6 format: 2001:0db8:0000:0000:0000:ff00:0042:7879)
Instead of using the join session function, i used the console command (4-finger tap or blueprint node): open [listen server’s ip4 address]
that worked. you could give it a try, good luck
also in DefaultEngine.ini :
[OnlineSubsystem]
DefaultPlatformService=Null