iWork does not allow the use of alternate ports for http (5005) and https (5006). So if your port 80 is reserved for webserver, and your NAS does not allow the use of standard https port 443 then you are in trouble.
I found a link very informative here:
hi guys, thanks to you I found a way to fix the problem I had with Iwork and my webdav (a synology NAS) ..
I simply forwarded port 443 external to 5005 (my nas webdav internal port)
this way Iwork "works"
That means, in your NAS, you still set the http port at 5005 (and/or https port at 5006). But in the router, you set up virtual server as such:
Port public: 443
Port private: 5006 (or 5005, matching your choice of http or https)
So in the iWork applications (Pages, Keynote, Numbers):
1. Press "My Documents"
2. Press "Copy from" at the bottom of the screen.
3. Choose "Copy from WebDAV"
4. Server address, enter "https://your-ip.com" (no need to specific directory here, port 443 is the default value, so you can omit this)
5. Then enter your username and passwd, press "Sign in".
6. The prompt "The certificate for this WebDAV is invalid. Would you like to continue?" should pop up immediately. Press "Continue".
If everything works out correctly, you should see your NAS files directory now.
Then you don't have to pay Apple for the iDisk to enjoy copying to and fro wirelessly.