I checked your website with my iPhone SE and I could replicate the issue you are experiencing. To make sure if this is not a core theme issue I setup somehow the same case scenario on my installation using X with the same version that you have:
If you tap the first menu and the submenus using your iPhone you will see that the menu expansion works with no problem. That shows indeed there is a problem on your installation and not the theme itself.
Now it is important that we test the case in details following the steps below. But it might cause the website to go offline or view incorrectly while testing. That is why we urge you to move your website to a staging server so that our testing will not affect the live website. We suggest that you contact your hosting service provider to get a help to create a staging website.
If you are interested in doing the troubleshooting steps yourself you will need to follow this way:
- Ensure everything is up to date according to our version compatibility list here. Please follow the best practices when updating your theme and plugins. Click here for more information.
- Test for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
- If you're using a CDN, please clear the CDN's cache and disable optimization services. You seem to be using the CloudFlare which you need to disable so that you can test the case.
- Increase the PHP Memory Limit of your server. Click here for more detailed information and how to increase the PHP memory limit.