I’m sorry I didn’t see this until now.
The realtek nics can be a real pain to deal with.
The undionly.* uses the built in driver bits in the nic. The ipxe.?pxe uses the iPXE kernel’s built in drivers. That is why you had better luck with the ipxe.?pxe kernel. There IS a realtek specific kernel, but the ipxe one is probably a good choice here.
Now I can tell you if your computers are uefi based (not legacy/bios) then you will need to use a different boot kernel for the FOG menu. You will need ipxe.efi. If you have a windows 2012 dhcp server or a linux dhcp server you can configure it to automatically send the right file name based on what the target computer is.
by using the ipxe.kpxe it initialises the hardware and starts booting. I reach the FOG-menue when the hdd is disconnected. When the hdd is connected it starts booting windows.
This doesn’t sound like you have the network adapter first in the bios then the hard drive. I can say for my company I have the hd drive only defined in the bios for booting. When a tech goes to image a computer I require them to be in front of the computer and press the F12 key while booting to get the boot menu. From there they pick the network to boot from.