Hi gabberkopf!
I use the Apps iTransfer and FileExplorer on my iPhone 4S.
iTransfer is free. It allows FTP Access to shares which are setup up for that. Files can be up/downloaded in a bulk. Photos directly from/to the iPhone's camera roll.
FileExplorer costs a few bucks. It grants access to all network shares from your DNS-320L directly. Also bulk up/download possible. Photos to/from camera roll as well.
There are some more available, but the above 2 I like most.
For local access from inside your home network you enter the IP address of your NAS for both, FTP and network shares.
For access from a remote location through Internet (WiFi or 3G) I set up a dynamic DNS service, so that I have a "real" internet-address. The DynDNS is set-up inside my DSL router (FritzBox) and with port forwarding it goes to the DNS-320L. I don't use its own internal DDNS, but this may work as well.