Both of them are plugged into wall sockets?
Just reset them and test in the same room.

Ada wrote

Both of them are plugged into wall sockets?
Just reset them and test in the same room.

Yes, they are. No additional plugs or cables: 1 PL adapter = 1 exclusive, reserved wall socket for it.

In the same room, they are working fine. I want to use them in different floors: in this case, I'm getting the Powerline LED blinking on one of them. In the same room they both blink.

I've reset them, changed the “network name“ many times with the Powerline Utility.. with no effort.

Internet works even without the Powerline LED on (because the Ethernet LED is blinking in both), but I'm experiencing issues this way. For example, I'm getting a strange “buzz“ on my (wired) home phone line.

I've not tried to pair them using the reset buttons because this has driven me to an impasse.

My diagnosis ends here. Please note that the different floors belong to the same electrical panel, no other electric phase, voltage or connection.

What can I do?

If they work in the same room, it means that they have been paired successfully. But they cannot work in different rooms, it means that your wiring in the house is not proper for powerline adapters. But if possible, you may replace them or test them in different environment to verify.