I think the better long term solution here (at least as a start) is to locate the router more centrally to the area(s) that use WiFi the most.
After that, you can change channels (1, 6, or 11 and 13 if available in your area and clients) and after rebooting all network equipment (router and at least the laptop you're testing with), see which gives you the best throughput and lowest latency.
Also, change the channel width to 20MHz for the 2.4GHz band and change the Preamble to Short.
If you've upgraded the firmware and did not do a reset at any time in the past, it is time to do a full reset to factory defaults (via the gui) and then manually and minimally configure the router to secure it and connect to your ISP (do not use a backup config file and I would also recommend to use new ssid's too).
As asked, what firmware are you currently running? I would recommend to use the latest RMerlin firmware or one of the firmware based on his (john9527's or hggomes forks).
Locating the router centrally is the best long term plan. Testing which channels you need to use for best performance is then the next thing to do (and do not rely on utilities like inssider to show you graphs of signal strength and number of other wireless AP's. In the end that is meaningless to the real world performance your network provides. Simply change to each channel and decide which is the best option, today).
After doing the above, you may still need to upgrade the router itself.
The RT-AC68U would be a good router and will increase range and throughput over the RT-N66U (especially with the same firmware forks listed above). But if you want to use a router and AP setup, I would recommend the two RT-AC56U's instead (or just one, of course to supplement your existing RT-N66U) for close to the same total cost and the ability to run the excellent RMerlin firmware and the forks.
Note that hggomes firmware fork will give you the best range and throughput possible, but it will also possibly boost the power output to over legal limits depending where you live.