Orbi RBR20 And nighthawk R7800 set up of network/suggestions

Hi

i am hoping someone here can guide with my best options. 
I have an Orbi RBR 20 and 2 satellites.  And I am having issues with the WiFi extending to the basement from the second floor where the main orbi sits, modem is on the second floor. 
someone suggested connecting A second wired router directly to my modem to the basement essentially creating a second "WiFi" area.  I also have a nighthawk R7800 which in theory I could do this with.  but it seems that everything I read here says don't do this.

could someone  please suggest is there anyway to use the orbi rbr20 + the nighthawk r7800 to increase the WiFi area I can cover in the house  ?   

thank you in advance for any suggestions.