FTP will be not usable on a iOS device, at least without jailbreaking.
iTunes sharing does not work even over VPN to home network. HomeSharing seems to be conciously restricted by software (tried & tested) to local LAN segment.
As a DNS-325 user I am able to connect to my NAS share over public internet using D-Link's mydlink Cloud app. Unfortunately, only listening to music works for me. HD movies do not play over WAN.
For you, as DNS-320 user, D-Link Cloud is not an option, unfortunately.
One possibility that works for iOS is to open WebDAV share on NAS and use WebDAV-capable player on iOS, eg
GoodPlayer.
Although GoodPlayer is also able to connect to SMB and UPnP servers directly from iOS.