Ideally, you'd get a firmware update from your NAS vendor that removes the Flash Player dependency.
Short of that, you'll need to use the Enterprise Enablement features to allow the UI for your NAS on your Intranet.
https://www.adobe.com/products/flashplayer/enterprise-end-of-life.html
https://www.adobe.com/devnet/flashplayer/articles/flash_player_admin_guide.html
You might want to think about making a VM that's dedicated to administering your NAS that has a build of Flash and a browser that will run it, as most browsers will be dropping support for browser plug-ins in upcoming releases.
See the original announcement from 2017 for more details and background:
https://theblog.adobe.com/adobe-flash-update/