First, you must understand how an extender works. You will need to add this extender in an area where you have minimum 2-3 bars. After you have confirmed that this location is prime for the extender then you can take a look at the cameras and see how the signal strength is, the recorder will see the cameras on the network, be sure to have the wireless network be the same as the NVR's network, some routers will create two different networks one LAN and another WLAN.
Also, when it comes to anything that uses RF it is prone to accepting noise, just because you get 5 bars, it may not be an optimal signal. It could be for example 30% signal and 70% noise.
Netstumbler is a free windows tool to see the signal to noise ratio.
If your router is not supported by Netstumbler, then try Kitz - DMT Tool
We do have some AP's that will provide you with a better coverage. TP-UAC-PRO