I have tried to join about 15 different custom match servers and they all say that connections been lost and then it puts me back into the menu. I have tried restarting my xbox and I have closed out halo 5 about three times and it still tells me that connections been lost whenever I try to join a custom match.
Please contact me if you know how to fix this.
Unplug your console from its power source for a few seconds and try again. If that doesn’t work, lookup how to delete your temp game data for Halo 5. Hopefully one of those two will work for you.