Hi,
I just visited your website using an iPhone and I am not having any issue navigating with responsive menu on your website.
The responsive menu is working properly.
This is a screen-cast of my visit on iOS Safari browser. https://drive.google.com/open?id=1QJRKo9o3CQ8GsfCt0mZz0jePWMVfwofQ
This is a screen-cast of my visit on Chrome for iPhone https://drive.google.com/open?id=1wporc7KmNzO_-aUlWJyOb46oLsveyq0V
I think you may have a problem touch triggering the menu. I advise you to consider increasing the menu button size, so that it increases the touch-trigger area. The option is in Button -> Container Sizing
Regards,
Kriti