Hello  @wtakumi 

As range extender and router are on different stairs , for better performance would you consider wiring  Range extender to router via Ethernet cable and use it in Access Point Mode. 

Range extender received the Wifi speed and transit the signal to network client, so the speed of range extender is usually slower than router's Wi-Fi, thus RE is mainly used  to extend Wi-Fi signal in a Wi-Fi dead zone. If you wish to get faster speed, wired connection may help in your case.