500D Transparent mode & no WAN port?

Hello everyone,

As the title say my objective is to set up my newly arrived 500D in transparent mode between my Internet-Gateway and Core-switch. (Internet)<===>(GW)<===>(500D)<===>(SW)

First thing I noticed when I got the FW out of the box is that there is no WAN port like other models i worked with, I connected a laptop to the MGMT port and gained access to the web GUI. then connected one of the Ethernet ports to the network, just to give and the 500D internet access to register my product and update the firmware, I tried setting this port with a manual IP and also DHCP but still keep saying "connecting..." am I doing something wrong?! do I have to do a different configuration to transform one of those ports to a WAN port??