Synology DSM 6.2 on DS418play: Synonet not waking 2017 iMac

Question

I am trying to use the synonet command to wake a 2017 iMac running on the same local network prior to performing an rsync backup.

I have configured the iMac to 'Wake for Network Access' in the Energy Saver settings, and I have created a scheduled task on the DS418play to execute a user-defined script consisting of the following command:

synonet --wake ma:ca:dd:re:ss:xx eth0

The task is run as root, and completes normally (exit code of 0), but the iMac does not wake.

I have ssh'd into the DS418play as root and run the command from the command line with wireshark running on the iMac. The iMac does not appear to be receiving any "magic packet" from the DS418play.

Has anyone experienced a similar issue? Any help appreciated.

(As a workaround, I've set a schedule within the Energy Saver settings on the iMac to wake it up 5 minutes before the back-up job runs, but I'd like to get the WOL script working)