I was able to set up my HA as a slave connected to my Synology master UPS server which is connected to my APC via USB. Once you add your HA device IP to the Permitted Synology NAS Devices, you can configure HA NUT as follows:
devices:
- config: []
driver: usbhid-ups
name: ups
port: auto
mode: netclient
shutdown_host: 'true'
users:
- actions: []
instcmds:
- all
password: <synology ups password>
username: <synology ups user>
upsmon: slave
remote_ups_name: ups
remote_ups_host: <synology IP>
remote_ups_user: <synology ups user>
remote_ups_password: <synology ups password>
You can set your UPS user and password in your Synology /usr/syno/etc/ups/upsd.users, something like this:
[monuser]
password = secret
upsmon master