Mac tries to download files using trnsmission
Download and install the Synology Drive Client utility from Synology Download Center. On macOS, we suggest using APFS file system for external drives in order to prevent Tick the Enable SSL data transmission encryption checkbox. self-signed certificate, or someone may be trying to intercept your connection. The easiest way I found was to select all torrents in Transmission, then go to the menu >> Verify Local Data and torrents should find your files in new folder.