[2019] Cannot connect to my friend's co-op game

Hey @Rebekah

Welcome to our community.
Please also make sure to have your ports open when hosting and/or trying to join the game.

TCP: 3074.
UDP: 88,500,3074,3544,4500.

We would also recommend giving the Dedicated Server app a try, as it removes some of the limitations of the coop game.