Custom Multiplayer Maps

Hello,

I am still new here but i have been searching for an answer to this for 2 days now. And i am confident enough there is no post about this yet.
I have a handful of friends that decided to get together and all of us got the game with the latest sale. Strange thing is that I can only be the host, as for some unknown reason if any of the other host they do not show up on the server list. But that is not the issue i need help with.
We have been playing for a couple of days now and really would like something different from the default multiplayer missions.
I have dived into making our own, and followed all the guides i could find on it. Exported a custom map. Yet it does not show up on the list. I then proceeded to download other custom maps. Which presumably work since they have downloads in the three digit numbers. Yet those don't show up either.

What do i need to do to get custom multiplayer maps to show up?
Is there a mod required?