Maybe I am missing something here - is the Apache server on the same LAN as the DNS-323?
- if it is, then you will not be accessing the DNS-323 from outside the network.
- if it's not, then I don't see the point in what you're trying to do.
You seem to be missing a couple of key concepts - http is not a protocol normally used to access stored files - and at least with the current firmware (1.06), you cannot access data on the DNS-323 that way, you have to use either ftp or CIFS/SMB.
So - if you have a program on a computer in your network that tries to automatically attach to a folder in the DNS-323 and display it's contents in a webpage - you would NOT be accessing the DNS-323 via it's web interface, but via, most likely CIFS/SMB (this will be determined by the program).
It is not uncommon to have a web server access other local servers for content to go into a webpage - it may be unusual to do it with low end gear like the DNS-323, but it is one of the building blocks in e-commerce installations, a so called "n-tier server architecture" where the webserver is the first tier and merely serves the webpages, the content of the page is generated by an application server (second tier) and there is a database server hosting SQL databases forming a third tier - and - when I say the webserver is the first tier, that first tier could actually be a few dozen servers, linking to multiple application servers in the second tier and then a cluster server for the database backend.