2FA, iOS 12 displays grey box
OS has configurable content restrictions that can potentially prevent the Duo Prompt from displaying correctly.
How do I resolve Duo Prompt display issues related to iOS content restrictions?
If you see a gray box instead of the Duo Prompt on their iPhone or iPad, check whether content restrictions are enabled.
For iOS 12:Navigate to Settings > Screen Time > Content & Privacy Restrictions > Content Restrictions > Web Content.
To completely disable content restrictions, uncheck Limit Adult Websites.
If you do not want to fully disable content restrictions, you can add duosecurity.com to the whitelist within the Content Restrictions page on the iOS device. This will allow the Duo Prompt to display even if content restrictions are enabled.
Full article is here: https://help.duo.com/s/article/3710?language=en_US