RAX78 WAN port keeps asking for Dynamic Address from gateway/router/firewall

Help! I'm just installing this RAX78 in place of the R8000P. The Internet port is set to Dynamic. I set a reservation in the gateway/router/firewall so that the RAX78 will have the IP Address that I want it to have. When I connect the WAN cable of the RAX78 to the LAN port of the gateway/router/firewall, it acquires the reserved IP address. But a second later, it loses that IP and re-acquire the same IP again. It keeps doing that preventing the RAX78 to provide "good" Internet connection to my devices connected to it. The Logs on the gateway/router/firewall confirms the repetitious giving of the reserved DHCP IP address to the RAX78. I thought it was the IP/MAC binding of the R8000P because I used the same IP address but rebound the MAC of the RAX78 to the IP address used by the R8000P. But it's not that because I deleted the binding and reserved it another un-used IP address and the IP/MAC binding is brand new. Still the same thing. So I set the Internet Port of the RAX78 to Static both gateway/router/firewall and RAX78 are communicating solid.

What is causing this? Could the WAN port of the RAX78 be bad? 

BTW, the RAX78 has the latest firmware.