I have exactly the same problem. And what's more, I find strange to have to do all this handle, because it used to work by itself without any fitting. I have the bad idea to rename the nas, (a Linkstation LS-CHL) and set some real passwords for the users. (I mean not "admin" & "password").
I tried to set ports in the router admin, and took a domain name at "no-ip". This is working, but point on the admin interface of teh NAS, not to the files !
Any idea ?
Thank-you !