The issue that I have been having is that I am unable to play the coop Combat Evolved campaign with my friends. We tested the other campaigns, and they worked just fine. I’ve been troubleshooting this for going on 3 hours now and it seems like a lot of people around the web have been having very similar issues.
I’ve tried just about everything on this list: Master Chief Collection campaign co-op not working? Here's how to fix it | PCWorld
The most common thing I’ve seen is check to make sure your NAT type is set to Open. Originally, my NAT type was saying that it was having issues with Teredo and that my server connectivity was blocked. Now it shows my NAT type is Open and my server connectivity is connected after doing a few things on the list in the link above.
It is worth noting that my roommate’s NAT type (who is on the same network) is shown as having the same issues with Teredo and that his server connectivity is blocked, but he was able to play the coop campaign with another friend of ours (who is on a different network).
Any help at all would be appreciated.