You have a mix of BIOS based machines and UEFI machines and you attempt to boot the incorrect type of Boot Program
If you have a mix of UEFI and Legacy BIOS machines you cannot use DHCP Scope Options to direct PXE clients to the Boot Program on the WDS server. You must use IP Helper Table Entries. For more information on configuring IP helper table entries contact your router/switch manufacturer.
The WDS Boot Program's for each type of machine are: