@YuiSS,

A Range Extender Simply Serves to extend the existing networks by repeating the network traffic. TP-Link Range Extenders have a High Speed Mode that will allow you to set a specific frequency to be used only for the backhaul network. I would assume that this feature would extend to the newest band.

In fact I found this in one of the images on the spotlight page: 

RE815XE takes full advantage of tri-band, empowering up to 96 devices with lightning-fast 5400 Mbps speed.³ With the 6 GHz band serving as its backhaul, all of your connected devices receive phenomenal connectivity.

Your router and Range Extender are also both OneMesh Devices, so if the Range Extender is setup in Onemesh mode, you may be able to take better advantage of all three frequencies