I think what you want is UPS Master vs Slave.
Provides information on configuring UPS service on your TrueNAS.
www.truenas.com
Set up client that is directly connected to the UPS via serial/USB, add remote user or enable remote monitor with default remote user id and password.
option 1: monitor users
Describes the UPS screen in TrueNAS CORE.
www.truenas.com
option 2: default remote user
either option: install NUT on other clients (if not already installed) and configure additional clients as UPS Slaves
Power Devices support
networkupstools.org