Friend attempts to join coop game, but ultimately fails

Hey, me and a friend have been trying to join each other’s games for 2 days now, but to no avail. We have tried port forwarding, but due to him having an incompatible router, we gave up on that.

This is the recreation with a clean, reinstalled game, and no mods activated:

  1. Both host and joiner launch game through launcher

  2. Host starts a single player/New game, joiner remains in main menu

  3. Host activates CO-OP for the new game

  4. Host presses shift+tab for steam overlay

  5. Host invites joiner to lobby through steam overlay

  6. Joiner, still in lobby, accepts invite, and is stuck in “Attempting to join game” for about 5-10 minutes.

  7. When that time has passed, the joiner’s game stops attempting to join, and the joiner is told that the game lost connection to the host.

The host gets no notification whatsoever, and it’s like nothing ever happened, and the joiner remains in main menu, unconnected.

I have the logs ready in a Google drive.

You guys are on same Wi-Fi connection?
If so then i think port forwarding is mandatory for it to work.

You guys can as well try to pick some official EMPTY server and just play on it together - chances that someone will join are small.

We are not on the same Wi-Fi connection, no.

Does anyone know a fix?