Note, by default, this would apply to your Personal Meeting ID. So, be sure to log into the Zoom application, then hit Schedule from the home screen of the app, then scroll down to 'Advanced option', open that, and check the 'Allow participants to join anytime' box. If you selected it only from the website, then it doesn't necessarily get applied to your meeting since it only applies it as a default setting for new meetings (it doesn't affect previously scheduled ones).
Did you select the 'allow participants to start meeting before host any time' feature from the website? Or from the Schedule popup of the Zoom app?