I think you are confused by the options.
You need to create a NUT server. Whatever you plug the UPS into (via usb) should be your server. This will read the UPS data over USB and the server provides this data to network connected clients.
If you use your synology box as the server, you do not need the add-on. If you use your Hass.io device as the server, you need the add-on.
With either option, you can setup the NUT sensor in Home Assistant to pull the data from the server.