In a nutshell, you need to configure your domain with an FTP CNAME record that points to your DDNS address. You also need to configure either your router or the NAS to login to your DDNS account, so that your external IP address is updated.
You will need to open up FTP ports on your firewall and port forward these requests to your NAS.
You might want to get your DDNS address configured and working before getting your domain to work too.