Friend cant join our game.

[DarkViolence]

Recently started playing Dead Island Riptide: Definitive Edition with my friends.
Initially played at my house, it all went fine. after 6 hours, they were at home and wanted to play again. we all connected except for 1.

So far what I noticed:

Host doesn't matter he will either get (You are now playing alone) or (Game auto shutdown)
he's connected to LAN cable with high speed and stable internet access (best out of all of us)
He can play alone with no issue his gaming rig is not too shabby.
Doesn't matter which character he is joining with, none of his characters can join
we even created all new characters at chapter 1 to try join him, doesnt work either.
Game has been reinstalled 4 times.

Until now he is still unable to join our games.

[Qlone]

First make sure that you have the same version, since Definitive and Vanilla are not compatible. If everyone has the same version, invite each other through the Steam chat, since the lobby sometimes does not show you your friends and prioritizes players who are in the same level or chapter of the story.