Plutonium

I tried to follow a tutorial but i think I failed and did it wrong, it still doesn't work
This is strange because it used to work a few days ago

Yes the host is already in game
No we have not tried because we have no idea on how to do it, i will do some research
Thank you for your quick answer

Me and my friend try to join each other and it just says "joining session" for a brief moment and then it disappears for zombies. Both of us have UpNp enabled. We also followed the instructions in this video https://www.youtube.com/watch?v=cstpHPdscsw but it still doesn't work. Any potential fixes?
I know this post has already been made but the fix doesn't work anymore