Cannot find Device when trying to access system using WAN

this is my last ditch effort to try to get the cheap China made system to work and I know I'm close. I can access my surveillance system using LAN IP address. But when I'm on my neighbors wireless network and I try to access it using WAN IP address it gets to video login screen and I don't have an password set up yet and hit OK, it just says cannot find device. In the LAN IP address I get that login screen and hit ok and brings my 4 cameras right up. I have Time Warner as my internet provider and they only offer a dynamic IP address so I've set up an free account with DYNDNS and have set all of that up with my Linksys router. I've been trying to get this to work for over a month and even had a security system guy out to my house yesterday and he just said the same thing, that I am close but he didn't know why it couldn't find the device. My port forwarding is set up as 80 (the suggested default my dvr instruction say to put it at). Please provide me with any advice or things to try before I throw it in the garbage.