Sometimes when joining a realm, you have to go out and back in to get into the realm.

Sometimes, when you join a realm when no one's been online for at least a few minutes, it won't let you join the realm, stuck on the 'Connecting to the Realm' page forever. After about 15 seconds if you go out and back into the realm, it connects almost instantly. Has been a bug since at least mc 1.14-1.17