Hi,
Welcome.
If your message is not only to advertise about filen.io services  
  
 ... (so with risk to be removed)
As I understand filen don't have a terminal or programmatic client for Linux ... so can't be integrated easily in NAS
filen have a desktop client, but NAS have a limited desktop manager (Asportal) so not sure it's possible to use desktop client on it (no taskbar, no fork, full screen only, not all libraries generally provide with desktop manager.
... eventually with an alternative to Asportal (the two hereafter support appimage packages)
... ... Linux station
... ... myHD APKG (replace Asportal with openbox/idesk env.)
filen have a mobile interface ... so NAS is not concerned.
Last Filen have a Web interface, but it's not clear, if it's a server (so potentially installable on NAS) or just an add-ons for browser like other cloud provider.
... in this case no integration, but always possibility to access shared resources from the client (Linux, Windows, Mac, etc.) and browser executing the add-ons.
It's not easy to test, no free pricing plan, no large free test period, only 4 pro I, II, III, X plans ... some expensive and recurrent (monthly)
So difficult to have a complete and correct response to your question, but ask Asustor support.
Philippe.