Trying to play this game with a friend. We have both reached the point where coop has become available.

When searching for friends (in game), they can see me online without any problem, but when I search in game all I see is "you currently have no online friends". However on Steam itself I can see them both online and showing as playing the game.

I am unable to invite them or join on them in game because of this problem, and there is no option to invite directly through Steam. They cannot see any option to join on me, but they can invite me (through game) and the invite appears in Steam, but when I accept it nothing happens (no messages or changes in game to indicate any attempt to join another game).

I have already tried some advice I've already found online, specifically: Running Steam as admin from both sides, setting both Steam clients to the same download region, and making sure the game has an exception set on the firewall on both sides. I've have also tried multiple restarts of the game as even this was suggested. However none of these have resolved the issue.

Does anybody else have any advice to offer to solve this problem please?