Setting up the proper shutdown scripts was a lot of config. This documents everything I needed to do to get it running.
The Synology NAS is the NUT server as the UPS USB cable is plugged into it. It's set to alert any clients subscribed to it when the UPS connected to it is in low battery. This all applies regardless of what machine you have a NUT server on, does not have to be Synology.
The UDM Pro does not have the ability to listen to a NUT server, so I've setup the Proxmox server to initiate the shutdown command to the UDM. I was able to do that by:
- Installing the NUT client on Proxmox
- Enabling SSH on the UDM Pro