Monica, I have some time before it is lights out at my house, so some more feedback for you. Radio signals are not a constant signal strength; they fluctuate constantly and can be affected by weather conditions, sunspot activity; you name it. Also, in the case of the app, you will see the graph rise and fall each time the app re-scans every few seconds for signals. You can see these signals move by using Signal meter view. The signal can also rise well above -30 dBm for short bursts, and I just ignore that anomaly. It happens on all the signals I see near me. You are looking for the most constant average range of signal strength over a period of time, and it can vary more or less at different hours or days.
There are steps you can take to improve things before you feel you need a range extender. One is router placement. Ideally, a router is best positioned in the center of a home and on the highest living space floor unless there are say, three floors, then middle floor may be best.
In practice, a router should never be on a floor and under a desk. Better to be on top of desk or table or even a shelf. Antennas should be as high as possible/practical. You have a few antennae on your router, and router user manual suggests the best arrangement of these.
My router is close to the center of our home near where people and gadgets hang out. In my house I also have what is called a "CAT5" box where phone, cable signals for TV and modem come in and 4 network wires for plugging the 4 wired rooms with ethernet sockets/plugs into the back of my router. This happens to be the master bedroom walk in closet. Router and modem are on the top shelf up against the inside wall facing center of house. I'd rather not have sheet rock wall as a barrier, but I am not going to cut a hole there to remove the sheet rock! Nevertheless, I'd say 80% of home is covered well; it is the rest of the home that needs the boost from range extenders. I'd rather not use them, but the loss they can impart to system is overcome by the gains they can provide elsewhere so I use them midway between router and other end of house and for downstairs improvement.
Being isolated from neighbors makes selecting a router channel easy for you. Always select CH 6 permanently if you have the luxury. It's the "sweet spot" in the home wi-fi band; right in the middle. You have, and I also have it available, and I use it. Most people near me using 6 are far enough away that their signals are weak enough to not be a detriment to my wi-fi.
Most of us have N band devices today, and the old G devices are gone. Problem is, routers come shipped to work on all modes, so I have my router adjusted to work on N Only, and not waste power on unneeded modes. Security for router should be set for highest WPA2 and AES encryption. Channel bandwidth should be set to 20MHz, and not a combo of 20/40 MHz to focus router power directly on the channel selected. And, check the power level of signal; put it at highest setting; you are not going to cause interference for anyone nearby!
Go through this checklist to make any modification to fine tune the router. Have fun!
Mike