I'm not certain what it is you need help with here - you only need one dynamic DNS service - take your pick of DLinkDDNS and configure it on the NAS - or - whatever the router offers and configure it on the router - I would say handling it on the router is preferable, but either one can work.
Now - lack of a dynamic DNS won't prevent you from connecting to an ftp server, it just makes it more challenging - and you haven't told us what errors, if any, you're getting when you try to connect, you also haven't told us what ports you've opened or how the NAS is configured so your problem could range from an incorrect configuration on the ftp client, the router, the NAS or all of the above, to your attempting to connect to someone else's ip address whilst your, perfectly configured installation, sits waiting for you elsewhere.
The basic steps are ...
1 - Assign the NAS a static LAN ip address.
2 - Configure the ftp server on the NAS using the static LAN ip address.
3 - Confirm ftp access on the LAN.
4 - Configure port forwarding on the router.
5 - Confirm ftp access from the internet using the router's WAN ip address.
6 - Configure dynamic DNS (on the device of your choice).
7 - Confirm dynamic DNS functionality.
8 - Confirm ftp access from the internet by URL.
Please note that confirming access from the internet is underlined - attempting to confirm internet access from within the same LAN as the ftp server may or may not give valid results, depending on your router.
The above, by the way, assumes that you have some familiarity with the use of ftp, at least from a client stand point, and that you know how to use the ftp client of your choice - for basic testing I prefer to use Microsoft's CLI ftp client, primarily because it does not attempt to interpret error messages that the server may send.