If the "MAC Address" is disabled in the Gateway Node (default configuration in GUI), the source MAC address will be "00:00:00:00:00:00" in the DHCP offer and DHCP reply messages, which are sent by Wifi Offload Gateway Node. You will have to enable "MAC Address" in Gateway Node manually, then the Wifi Offload Gateway Node will send DHCP offer and DHCP reply messages with the non-zero source MAC address.