I am using 5GHz uplink for my tp-link RE220 by manually setting it under the advanced menu.
Therefore, the extender extended network broadcasts only on 2.4GHz, but there is no option to select a channel on the extending network radio. It is set to auto and chooses channels as it pleases.
Please add it as a feature, there is no good reason to lock the extending radio to auto, since it is not a repeater.