Can you provide a diagram or at least explain what your situation is.
Why would your wan port coming from the ISP be the same physical port to your LAN???
Perhaps you have a switch between your ISP modem and your router??
In that case assuming the switch is vlan capable, there should be no issue with running two vlans on the same physical port!
Mkx is bang on. All the info you need is included in that post....
I'd rather manage rats than software. Follow my advice at your own risk! (Sob & mkx forced me to write that!)