You’re having this problem because your custom domain points to a custom page that embeds your Netlify site in a frame, and said custom page does not have the viewport meta tag in its source code to let mobile browsers know that it is responsive. While the pages on your Netlify site do have this meta tag, it is ignored inside a frame.
First, your custom page is using the old HTML 4.01 standard from 1999, using the obsolete frameset and frame elements. Also, this framing solution prevents linking to other pages of your site from your custom domain. You should consider deleting that page altogether because of these issues.
As for a proper solution, I would recommend setting your custom domain as a domain alias in your Netlify settings, and modify your DNS records to directly point to your Netlify site. You can find more information about this on the Netlify Docs section about custom domains.
Hope this helps!