I don't have a daul-wan setup at the moment, but I did have in the past, using an ER-5-POE, and it worked with Plex. Let me find an old config from that router. and post it here for you to look at.
I guess you are accessing remotely with a public IP from your ISP?
Have you added the bits on the end of the URL? If I tried to access via http://xx.xx.xx.xx:32400 on it's own, it never worked, and I would get a 401 Unauthorised message.
However, if you access with http://xx.xx.xx.xx:32400/web/index.html#!/dashboard it works, or does for me.
Could it be this?
I guess you are accessing remotely with a public IP from your ISP?
Have you added the bits on the end of the URL? If I tried to access via http://xx.xx.xx.xx:32400 on it's own, it never worked, and I would get a 401 Unauthorised message.
However, if you access with http://xx.xx.xx.xx:32400/web/index.html#!/dashboard it works, or does for me.
Could it be this?