No Add Apps button on dashboard

These devices do not officially support transmission. But there are plenty of guides how to do it in this forum.
Such as this one [APP] Transmission various versions for firmware V4+ (05/2020)

FYI. Any NAS you might get regardless of vendor, it will come with a light flavor of Linux. So in order to install any apps, make sure you either download a specific version for your NAS or compile your own package.