Access modems behind ASUS router (Dual WAN)

[ColinTaylor]

The ASUS DDNS seems to work ok, in that if I type in the DDNS address, it will bring up the modem on WAN2 DNS error page. This means the DDNS is redirecting to my public IP on the primary WAN fine.

I don't understand what this "DNS error page" is that you're describing. Can you post a screen shot. You should be testing this from outside your network, it won't work properly testing from inside.

TCP / Port 80 / 192.168.4.20 (IP assigned by the modem to ASUS router)

As a general point it is strongly advised that you do not allow remote access to your router's admin interface over HTTP or HTTPS (if that's what you're trying to do). The router's httpd server has a long history of security vulnerabilities that have been actively exploited.

You said in post #18:

2. Add DMZ for the router 192.168.1.1

Are you talking about the DMZ configuration on the Asus? If so remove it.