Ok, you type in HTTP://DOMAIN.COM
... and everything works correctly, I'm assuming.
However, when you type in HTTP://18.104.22.168
... you get the "It Works" default apache page.
Short answer - it can't look up 22.214.171.124 in the server to resolve it to your website
Longer answer - because of BIND, your server doesn't know how to look up 126.96.36.199 in its DNS tables, so if you type in DOMAIN.COM, the nameserver points to your machine, in which it resolves DOMAIN.COM to /var/www/clients/clientX/webX/web/index.html (or php if you prefer).
The reason it cannot resolve 188.8.131.52 to DOMAIN.COM, is that ISPConfig can be configured for multi-domains. So if you had DOMAIN1.COM and DOMAIN2.com, they'd both point to your IP address, but on the server, be two different directories.
In this case how would it resolve 184.108.40.206? To DOMAIN1.COM or DOMAIN2.COM?
In fact, the default Apache2 has its root document directory at /var/www ... and if you look in there, there's a INDEX.HTML file sitting there.