iOS has configurable content restrictions that can potentially prevent the DUO Prompt from displaying correctly. If you see a gray box instead of the Duo Prompt on their iPhone, iPad, or Mac please follow the steps below for your device.
If the user does not want to fully disable content restrictions, you can add duosecurity.com to the whitelist within the content restriction page on the iOS device. This will allow the Duo Prompt to display even if content restrictions are enabled.
For users on iOS 12 or later:
For earlier versions of iOS:
For Users on a Mac with macOS Catalina 10.15:
For earlier versions of macOS: