Preamble: In my company we add a new Vodafone's router that's already compromised my working DHCP server. That's because Vodafone refuse to disable the DHCP server of their router. So I figure out to split the lan in 2 subnets and put a firewall in the middle to block the DHCP server.
The firewall I'm using is an old Zyxel Zywall 10w but I think it's ok for the purpose. It has 2 ports (1 LAN and 1 WAN). I configured all the firewall rules but there is a problem. The WAN led isn't blinking so it doesn't work.
I think it's because it want a WAN attached to the WAN port, I'm connecting another LAN. Is it the problem? There is an option to set this? (maybe encapsulation, rip, etc etc)
You're not actually doing anything that wrong.
From a router's point of view, there's nothing really that different between a WAN and a LAN, it's just a different block of addresses, but it's all Ethernet (or TCP/IP, depending where you're looking from) really.
Things I'd check.