First thing : Welcome !
I had the same issue and made a topic about this. You have multiple options :
-If you can you could set up a private server and invite your friend. Playing on a private server means that there will not be any thether distance and so your friend don’t need to stay close to you as in the co-op mode. Unfortunally, you’d need a spare machine to run the server.
-To play the co-op mode, you need to open some firewall ports, if you want i can send you the ones that you need to open. Playing on co-op means that some things will be special :
. If your friend die, he’ll spawn back next to you.
. If you die, your friend will get teleported next to you.
. Your friend won’t be able to do stuff if he’s to far away from you (you can max that distance, but the radius is around one case around you).
. I don’t remember anything else but if you look on the forums, you’ll see some stuff
I post here my topic where Kearsan helped me a lot. If you have any questions ask me !
I hope I helped you a little c: