Don't scroll the IFrame page or its content, scroll the parent page. If you control the IFrame content, you can use the iframe-resizer library to turn the iframe element itself into a proper block level element, with a natural/correct/native height. Also, don't attempt to position (fixed, absolute) your iframe in the parent page, or present an iframe in a modal window, especially if it has form elements.
I also suspect that iOS Safari has a non-standards behavior that expands your iframe's height to its natural height, much like the iframe-resizer library will do for desktop browsers, which seem to render responsive iframe content at height 0px or 150px or some other not useful default. If you need to contrain width, try a max-width style inside the iframe.