Hello, @Alessandro,
Actually, the issue on your site is because of that AMP plugin, when I checked your site on my mobile without AMP it is working fine, and when I checked your site on my mobile with AMP, it distorted and half of the content vanishes as well.
And with Custom CSS this is not possible as I can see there are many other issues on your site as well in the mobile version with AMP, so I suggest you to please disable the AMP plugin from the Dashboard >> Plugins >> ALL Plugins and then check back to your site after clearing the browser cache.
Also, check this article for a better reference: https://xstore.helpscoutdocs.com/category/112-amp-plugin
Regards 8Themes Team.