Setup as wifi repeater doesn't work - Setup as AccesPoint is fine

Hello OpenWrt-Experts,
I am new in this software - I had this repeater (TP-Link AC750) for serveral years but now I need stronger encryption than the firmware can deliver. So I made a trial with openWrt version 21.02.
Everything works as it should when I am using it as AP for both 5G and 2.4G.

But in the moment where I am adding a client Wifi things don't work as expected. I think it is because I probably misunderstand the function of interfaces. Do I have to define master and client in the same network. Currently I have configured all in network "lan (br-lan)". I made already several tests with wwan, but they all fail.
In the moment where I setup the client e.g. for the 5G network I even can't see the ESSID anymore on my wireless device (e.g. mobile phone).
Could someone tell me how to setup or share the screenshot of his repeater setup? Maybe I've just not found the documentation for what the relationship of devices and interfaces is.