I have the same issue on the Realtek 8168. But this is all under hyper-v. On my old pc, with intel NIC, everything worked fine, but now on my new PC with the Realtek 8168 I am running FOG as a virtual machine and a PXE client on. All on hyper-v. It looks like hyper-v inherits the properties of the host system, even with private networks. When using EFI it all works, just not on a legacy NIC.