Thanks for asking.
Upon testing the website I can confirm that mobile menu is not working and that is because you are using older version of X theme. You seem to have updated Cornerstone plugin to version 3.0.4, but you haven't updated X theme which cause this issue. Right now you are using X Theme version 5.2.5. Please update to version 6.0.4 and doing so should fix the problem.
Here is a update guide that you can refer: