How to access external DNS on iOS when it is blocked
The easiest way to solve this is probably to use an app such as "Weblock - AdBlock for iOS" that works through proxying content. In the app you create a redirect rule for your external web site URL and to the correct IP address of the website (port number 80 for http or 443 for https).

Another possibility is using a VPN app to allow you to punch through and access other DNS servers.