The APC UPS is covering the home server, as well as the switch and router, fibre optic subscriber connection, phone hub, and connected external disk drives. It is connected via a USB cable to the server to allow monitoring and reacting.
In order to monitor the UPS, install apcupsd with
sudo apt install apcupsd
After that, configure /etc/apcupsd/apcupsd.conf to have UPSTYPE usb (default) and empty the DEVICE entry (default is /dev/ttyS0 which is incorrect), then restart the daemon with
sudo systemctl restart apcupsd
We can get the current status with apcaccess.