use a VLAN to link the WAN port with the LAN port the set-top box is attached to.
My tv set-top box (Telenet Digicorder in Belgium) needs an external IP address. Without it, you’re missing certain functions, such as browsing the program guide further than two days, or planning recordings over the web. The Digicorder only gets an external IP address when it’s connected to the modem/router my ISP (Telenet) installs.
If you use your own router with DHCP enabled, you run into troubles. Everywhere on Belgian forums you will find that I should buy Powerline adapters to keep a direct line between the Digicorder and the Telenet router. Or I should drop the (silly) idea to have my own router… But where’s the fun in that?