Best way to deal with WiFi Dead Spots?

I have a Nighthawk X6 (R7900) that I am using with AT&T Fiber as my broadband provider.  I have a house with four floors and, as expected, there are spots in the house where the wi-fi signal is either non-existent or extremely weak; other than that, I am quite pleased with the speed of the wireless connections (and of course wired is blazing fast -- 800+ Mbps both directions).  Currently, I have the AT&T Modem (Pace 5268AC) in the basement and that is connected via Ethernet to a switch on the first floor and the Nighthawk is connected to that switch; I have disabled the Wi-Fi on the AT&T Modem -- based on the recommendation of the tech that installed our internet service -- and am using the Nighthawk to broadcast the Wi-Fi signal. The Wi-Fi signal is very strong on the first floor, in the basement, and in some areas of the second floor, but non-existent in other areas of the second floor -- including the master bedroom and home office -- and spotty but serviceable on the third floor. The home office has a wired ethernet connection so I have my laptop/desktop connected without really needing wi-fi, but I would prefer to have a wi-fi signal in there as well for my iPad.  The house is set up with home runs originating in the basement, so the connection in the office is from the AT&T modem and not from the Nighthawk router.  I tried an AC plug-in wifi extender (from another manufacturer) and that did not work well, so I will be returning it.  WIth my somewhat limited knowledge of networking, it seems like I have several options: 1) trying another plug-in wifi extender -- figuring I would try Netgear although I'm not sure manufactured compatability was an issue; 2) setting up a second router in the home office, which is adjacent to the master bedroom and therefore should solve the problem in there as well, or 3) using an ethernet (as opposed to plug-in) wi-fi repeater in the home office (I'm not clear if this would require moving the Nightwawk to the basement so that the home office is connected directly to the Nighthawk) -- and possibly additional alternatives that I'm not even considering.  Also, need to mention that I have DirecTV and am using the Wireless Genies and I did see some posts on another site about interference on the 2.4 GHz band between the Genie devices and wi-fi plug-in extenders.   Thanks in advance for any guidance/suggestions!