This hardware is really old, and is only a 4/32 device. While you are on the latest firmware that is physically supported on your device, that version (19.07) is EOL and unsupported. It has many known security vulnerabilities and should not be used anymore. It would be a good idea to start researching upgrade possibilities so that you can run a current and supported version of OpenWrt.
Since the device is using stock firmware, you will probably need to ask TP-Link for support. We don't know any of the details about that device and how it achieves its extender functionality.