The normal Linux way for DHCP to give direction for PXE boot is :
next-server 10.10.10.10; filename "/pxelinux.0";
However WDS takes a different approach as it does not have a pxelinux.0 file.
Instead it presents a program called wdsnbp.com.
Easy fix, we just replace the lines above with:
next-server 10.10.10.12; filename "boot\\x64\\wdsnbp.com";
Remember to restart the service :
/etc/init.d/isc-dhcp-server restart
UPDATE: Also have a look at this post to add support for UEFI:
http://tomas.solamail.no/2019/03/20/wds-uefi-bios-dhcp-options/