Second guy you talked to did NOT tell you to come here. Although the mistake can be done, this isn't the forum you're looking for that Xbox support directed you to. While we CAN help you with this issue, the guy told you to head to Halo Waypoint, 343i's forum.
In any case, it's probably an internet problem on your side. You can get in the game no problem, and other people can be searched up as you join a matchmaking lobby, but the moment the game starts matchmaking, you glitch out.
Maybe tell us your internet speedtest, and we could tell you more. Also, tell us if you have a usually open, moderate or closed NAT.