Zlaxhe, let me see if I understand what you are trying to do. You have an AS-604T that you are using as a server in one location but you have several NAS boxes at different locations that are outside your local network. If that is the case then you are trying to do what we do on a regular basis between our servers and storage system and our clients NAS boxes in different towns.
To do that we use either RSYNC or some custom scripts that are run by an FTP client that 'talks' to the NAS FTP servers. Most of the data transfer is from NAS to our storage but does go the other way when we test backup restore.
As far as I know the AS-604T has two FTP clients, FTP Explorer and NET2FTP and it should be relatively easy to write a script that finds the local net facing IP address of the NAS servers, transmit it to your fixed IP address, load it into the FTP client with a list of what to upload and then upload the data.