@gooses ok a couple of things here.
WDS uses proxydhcp requests. The is only relevent if your wds server is sending this boot information to your fog environment. This would be managed by your dhcp-helper service on your subnet router. If you are pxe booting into FOG OK then you can ignore this bit, I’m only posting it because I have seen conflicts with wds/sccm/fog in the same environment. This is because proxydhcp overrides dhcp option 66 and 67.
Now to pxe boot bios based computers you would setup dhcp option 67 to be undionly.kpxe. For uefi systems you would setup dhcp option 67 to be ipxe.efi or snponly.efi. You can not boot a bios based computer with a uefi boot loader, the same is true for booting a uefi based computer with a bios boot loader (which i’m guessing is the root of your issue).
The FOG Project also has a wiki page that describes how to configure a windows 2012 or later dhcp server to support dynamic pxe booting of both uefi and bios based computers. https://wiki.fogproject.org/wiki/index.php/BIOS_and_UEFI_Co-Existence#Using_Windows_Server_2012_.28R1_and_later.29_DHCP_Policy
The other option is to install dnsmasq on your fog server and let the fog server send out pxe boot info (only) on your imaging network. In this role your main dhcp server still issues IP addresses, just the pxe boot info comes from the fog server: https://forums.fogproject.org/topic/12796/installing-dnsmasq-on-your-fog-server One possible use case for dnsmasq is if you have a dhcp server that can’t be modified or one that doesn’t support dhcp dynamic pxe booting.