Question about APC Back-UPS Pro

You can only use the UPS locally, the PC needs its own UPS, unless you connect the PC to the UPS with a long mains cable (220V), then it will work

And what exactly do you want to do this for? The UPS should supply the NAS to prevent data loss and to stabilise the voltage. And to shut down the UPS cleanly in the event of a fault. You won't be able to supply the PC on the 3rd floor with power from the UPS anyway? So what's the point?

The software on the PC can only be connected to the UPS using a special USB cable (USB to RJ12). Unfortunately, it is not possible via LAN, even if the UPS has a LAN connection. For monitoring purposes, I installed the WinNUT software on the PC and connected it to the NAS UPS server.

If you have a Syno NAS, I would connect the UPS there and operate the NAS as a UPS server so that you can "operate" the other NAS as a UPS client