When an account on my Home Xbox tries to join a Realm, it says it could not connect every single time. However, I can connect to the default online servers provided. I've noticed this only seems to be an issue with accounts without an Xbox Live Gold subscription. The issue with this is, when sharing an Xbox Live Subscription on your "Home Xbox" allows everyone on that console to have access to online multiplayer features. I hope this information was helpful, best of luck fixing the bug and happy programming!