Your code is indeed correct. I have found it sometimes doesn't focus the element you've specified because of some conflict with the default focus behavior. To fix the issue and ensure the element you've specified is focused, just delay that call. It's somewhat fragile but that's the only solution I've found. You may want to play with the delay amount - the following is a 0.75 second delay.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.75 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
UIAccessibilityPostNotification(UIAccessibilityScreenChangedNotification, self.myFirstElement);
});