Getting past the Synology default web site when creating a web server

Because DNS doesn’t know about your site. Perhaps adding the site’s entry to your local ‘hosts’ file will help your computer find the site.

Although that will only help with your computer. Everyone else won’t know about it (which may be what you want).

If it is a public site associated with a domain, then you’ll need to get your hosting place to associate that domain name with your hosting IP. But if it is a private/testing site that only you (and your computer) needs to access, then adding an entry in the local ‘hosts’ file will be your first step.