but since I have CD on our server already I can’t get anything to boot to FOG over our network - it all defaults to CD.
Its not clear exactly what you are doing here. FOG doesn’t use a CD/DVD it boots over the network. FOG is similar to CloneDeploy or Clonezilla but different.
For FOG you will set dhcp options 66 to the IP address of your fog server and dhcp option 67 to the boot file name. The boot file name depends on what format your computer’s firmware is in. For a BIOS computer, dhcp option 67 is undionly.kpxe and for a UEFI computer the boot file name is ipxe.efi.
Once those values are set in your dhcp server then you pxe boot your target computer into the FOG iPXE menu.
I have a feeling since you referenced CD several times you might be doing something non-standard with FOG. Can you explain a bit more of what you are trying to do?