You could try the usual solution of a Powerline Bridge and a router configured as a WAP. It's basic networking so I'll give the search about the router as a WAP and stop there. Why? Because I've found extenders to be so hit and miss that I no longer use them.
Your SBG6580 -> Powerline Bridge -> other Powerline Bridge in a central location for the router as a WAP.
I am looking for a compatible wireless extender sub $100 to use with a Motorola Surfboard SBG6580 which is provided by my internet provider..Wireless router is currently located in basement rec room at one end of bungalow. In the dining room at the other end of house on main floor only get 1/5 bars and on the back or front deck 0/5..the house is not an open floor plan on the main level so signal will still have to travel thru some walls to reach outside or other rooms on main level