Unfortunately, no ideas come to mind, this isn’t anything I’ve dealt with in the past. Why is the page being delivered in an iFrame? Is there a mobile plugin getting or third-party service getting in the way? I’d start by testing with a default theme and all plugins deactivated. If you send a link, I can take a quick look.
It was not that that solved the issue it was something else a service I was using to display social media links to the content and that was putting content in iFrames (ridder.co) and this is why my readers could not view the content on iPhone devices. So best not to use it to avoid this issue.