This could be hard to troubleshoot without Safari. Safari can display the site's source code on an iPhone so you can troubleshoot CSS/jQuery issues on a desktop when the iPhone is connected to it. If it's a CSS issue, there may be another element laying on top of the submit button, or jQuery is producing an error that is preventing the submit button from working, or third, if you are using AMP, it's possible the jQuery is not active on the iPhone. Make certain Javascript is enabled on the iPhone under Safari's Advanced settings page in the settings app.