VPN issue on IPhone and IPad

We are facing a very weirded issue when connecting native VPN (IPsec) from IOS-based devices such as iPhone or IPAD that VPN is connected successfully but try to open the intranet-based link the site is not responding when using address with a name, however, when open with IP its open successfully and it looks like a DNS issue as VPN user is unable to resolve DNS, therefore, a website with DNS name is not openinng.

For Example: When i type http://helpdesk.abc.com it gets not connected however when I try to open 10.1.1.100 it opens.

The above scenario is working fine on Windows and Android-based devices (IPSEC).