I think you read, or implemented, only half of my brief suggestion. If you are getting the configuration page from the outside when you forward port 80, that means you're passing port 80 directly through to the NAS still on port 80.
My suggestion was to forward port 80 from the outside world to port 9000 on the NAS's local IP. In my Linksys router's configuration, I forward external port 80 to internal port 9000 on IP 192.168.xx.xx (the NAS's IP). If I passed external port 80 to internal port 80 on the NAS's IP, I would indeed get the configuration page as you mentioned.
The OP also mentioned he is behind two routers. In that case, one router would need to forward external port 80 to some chosen port on the 2nd router, and that router would forward the chosen port to port 9000 at the NAS's IP. Two layers.
Then comes the suggestion to make sure that the NAS is actually configured for web access, but I'm assuming that the OP already verified that it is available from within the LAN.