Connection lost error in campaign multiplayer

My friend and I keep trying to play the second campaign but every time we load in we get a connection lost error. All the proper ports are open, everyone has the multiplayer and campaign installed, we all just verified game integrity on steam. Nothing is working to fix this. We had the same issues with Halo 1 campaign until Halo 2 came out. This shouldn’t happen in a game that’s already been made.