A:
Sure. This extender should work with the R6400 router. But please read more before making a buying decision.
First of all, the BestBuy picture for this extender shown an old version (WN3000RP version 1). If you buy from BestBuy, you will get the latest version WN3000RPv3 (version 3) which is more compact and works better than version 1. The WN3000RP should work with most routers that support standard wireless-N (802.11N) which includes the NetGear R6400 router. However, it is not a good idea to buy this extender for the R6400 router. The R6400 router is a wireless-AC router which supports a faster WiFi speed: 450 Mbps at 2.4GHz band and 1300 Mbps at 5GHz band. The WN3000RP extender can only run at 300 Mbps max WiFi speed in a single band 2.4GHz, and therefore it can not take the advantage of the higher speed provided by the R6400 router. You should buy an extender that supports wireless-AC like the NetGear EX6100, or EX6150, or or EX6200 or better extender.