I have checked your website and found the problem you specify here. The Humberger menu is visible on specific breakpoints which have been overridden using the custom CSS code in the style.css file of your child theme. What you are trying to do, is required a few more customization like adding the collapsed class on that specific breakpoint to the dropdown menu, I would suggest you hire a developer who can assist you to do the customization or you can avail of our newly launched service called Elite service, where we offer complex customization.
Alternatively, you can create a custom Header using Header Builder which is exclusively available with Pro not the X you are using, and add the Humberger menu elements for the specific breakpoints and Navigation Inline elements only for other breakpoints.
I would suggest you upgrade to the Pro theme, please go through the process here: https://theme.co/docs/converting-from-x-to-pro
And also go through the following article on Header Builder.
And then you can add the following Navigation element for the desktop:
- Navigation Inline
And then you can add the following Navigation elements for smaller screen:
- Navigation Dropdown
- Navigation Modal
- Navigation Off Canvas
And adjust the Navigation by using the Customize > Hide During Breakpoint option to show/hide in the specific screen size.
Hope it helps.