Unless you actively set the powerline devices to have no security all the ones I have ever had came in pairs (or more) sets with a random password for them all. . . . and any units or pairs more I added needed to be 'added' to the existing password network to work with them. As others have said they work best on the same power circuit, reasonably across all circuits in the same house and not well on another supply Phase (which will generally be found in the houses on either side)
Ideally use the administrators tools and set them up with your own complex password