@Nabeel Did you configure your dhcp server to supply dhcp options 66 and 67? For dhcp option 66 that needs to be the IP address of your fog server. For dhcp option 67 that should be undionly.kpxe for bios based computers and ipxe.efi for UEFI based systems.
If you need to support both systems in your environment then once you prove that the above options work you can review this article: https://wiki.fogproject.org/wiki/index.php/BIOS_and_UEFI_Co-Existence#Using_Windows_Server_2012_.28R1_and_later.29_DHCP_Policy