Why not call the ISP, ask them what that devices does (i.e modem and/or router)? Why try to sneak around them? And ask them if there is a user interface that you can have access to? I know lots of ISPs that provide a router will provide access to the 'user' menus.
I tried powerline extenders, and generally speaking it sucks and I don't use them. Low bandwidth capabilities, and generally low performance / stability. Depending on where you live will affect the wiring in your building. In North America, our homes are wired twice. There are two 'hot wires', not one. Each hot wire is 120VAC, but each is opposite in the sine-wave cycle (some say 180 degrees out of phase but that is not technically correct) thus giving you the 220VAC for your stove, clothes dryer, and other heavy load appliances. What does this mean to powerline extenders? What this means is if you plug one extender into an outlet on "side A" and another extender into an outlet on "side B" there is no way for them to communicate directly, since the two sides two not touch each other directly. Thus there are only two 'paths' for the powerline signals to reach each other: (1) through the power company transformer that feeds your (and your neighbor) house, and (b) through the appliances I just mention. The transformers is a huge inductive/magnetic device, and the the appliances in your house have huge resistance currents going through them, so very harsh and 'unfriendly' to powerline signals. Unless your two outlets are on the same 'side' and wired on short runs, the performance is generally disappointing.