Yes, preferably buy your own router. The modem/router combos supplied here are not the best, do not provide the strongest wifi signal, usually have a single antenna, do not necessarily support the latest high strength/speed protocols (A/AC).
A router that dedicates two or more antennas to transmitting and two or more antennas to receiving works best. Locate the modem centrally. Wifi signals pass through concrete better at 90 degree angles than they do at oblique angles.
Range extenders within line of sight of the main router where possible. Chain them together (extender #1 connects to the router, extender two connects to extender #1, extender #3 connects to extender #2 etc.) to go down the hall, around the corner and up the stairs for example. Connecting as many of the extenders as possible to the router by ethernet cable is preferable. At the very least try to connect the first extender in a chain to the router by cable. Alternatively, run a cable to each floor of your house from the router. Connect an extender to the end of each of these cable runs.
Lastly, do not forget to turn off all the functions of the modem/router combo supplied by your ISP. No wifi, no dhcp services, no nothing other than to act as a connection from the wall to your new router. All network tasks, wifi and routing functions are to be performed by your router. Your router is connected to the now dumb ISP modem by cable.