Lobby Not Joinable is very annoying and It happens even when you play with your friends in the same WiFi, this is a bug that probably caused by the connection of servers, there are some solutions that had help other player to solve the problem, worth a try isn’t it?
1.Make sure everyone in your party has an OPEN NAT, make sure everyone’s party privacy is set to open (settings can be found in options)
2.Go social, party management and max players must be set at least higher than 1
3.Sometimes the party leader or lobby host will have an insufficient Internet connection to host the party or lobby. Try changing them or have another friend host the party or lobby to see if it fixes the problem.
We will update when we find or Treyarch has released any method to solve the issue, stay tuned.