@Cris13 From the things you posted it seems you have the exact same issue than others using the same NIC model. The issue is specific to that NIC. I have spend hours of work over weeks to look into this and digging through the iPXE and Linux kernel code - source code of both drivers are very similar to some extend but only the iPXE code seems to have the issue.
I was unable to work on this in the last weeks but I still hope we can somehow figure this out and make it work again. It’s just a matter of finding the right needle in the hay stack (more than 8500 lines of C code). Are you keen to get into this. I can give you instructions on how to compile and debug iPXE.
Just for reference:
http://forum.ipxe.org/showthread.php?tid=8296
https://forums.fogproject.org/topic/10261/macos-update-broke-ipxe
https://forums.fogproject.org/topic/10289/boot-from-usb-problem
https://forums.fogproject.org/topic/10043/netboot-problem-new-mac
https://forums.fogproject.org/topic/9269/imac-27-ipxe-boot
https://forums.fogproject.org/topic/9278/fog-1-3-0-pb-with-mac-netboot
https://forums.fogproject.org/topic/10810/multicast-error-with-fog-usb-new-imac
https://forums.fogproject.org/topic/10879/ipxe-boot-with-imac-late-2013