I keep having this issue, I click multiplayer, it establishes a quality connection, then I pick a hopper, and it tries to join but fails. Sometimes I can get it to work by trying again sometimes it will never work. NAT type is open, Xbox test connection checks out. (I am using an Xbox 1). I never have an issue with private online races. Once successfully into a public online race everything works great, it seems just to be a joining thing.