Importing music into your Doppler library from a NAS drive is slightly different on Mac and iOS.
Doppler for Mac
To add music from a NAS drive in Doppler for Mac you should first connect to your NAS drive from within Finder.
Once connected, you can add music to Doppler like you normally do.
You can drag and drop your files/folders from Finder into Doppler, or use the File → Import menu item.
When first setting up your library you can also choose a folder from your NAS drive to look for files.
Doppler for iPhone
To add music from a NAS drive in Doppler for iPhone you should first connect to your NAS drive from the iOS Files app
Once connected, you can add music to Doppler using the "Import from other apps" or "Import from Cloud Services" options.
After selecting one of those options, you will see a folder picker from the iOS Files app. Tap "Browse" at the bottom of the screen to show all of your options, and then select your NAS drive from the "Locations" list. You can browse and select your music from here.