@mlcomputers 

Do you still use your old wireless router in your current network topology (along with er605) ?

If you do, just change it's working mode to an access point - thus you'll avoid the double NAT for devices connected to the old roter.

Here's am example of how to configure an FTP server with Synology NAS.

  @mlcomputers 

The DMZ on your modem shoud be configured with the er605 WAN IP address. Describe your network topology in more details.

  @mlcomputers 

You gonna need some more ports opened on the er605 router.

FTP (File Transfer Protocol) uses two distinct channels: a command port (21) for instructions and a data port (typically 20) for actual file transfers.

In Active mode, the server connects from port 20 to the client, while Passive mode uses random high ports (>1024) to bypass firewalls.

There's a blue hyperlink in my first post - you can check how this was imlemented on a Synology NAS (in Passive mode).

  @mlcomputers 

For FTP server in Active mode, er605 Virtual Server should look like this:

If you want to use the FTP server in Passive mode, in addition to TCP port 21, you should enable the FTP ALG as well: