The following is a quick guide on getting basic status LED functionality working with TrueNAS running on the UGREEN DXP4800 Plus. Theoretically, it should work on all models (with some small revisions to the script), but I only have a DXP4800 Plus. :)
This guide is for cron job that runs a script to update the LEDs every couple minutes, but I'm sure the following can be modified for blinky LEDs as well.