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but which IP does then WiFi.localIP() return ?

and connecting STA to AP usually restarts SoftAP because it must match the channel used for STA. I guess that is a problem for your WiFi Manager as it disconnects clients

In my pull request, thelocalIP() method, in case of WIFI_MODE_APSTA, returns the address assigned by the router, but in fact it would be more correct to provide a second method that explicitly returns the IP address of the AP as happens with the ESP32 core for Arduino calling theIPAddress softAPIP() method, while I thinklocalIP() should always return the STA address.

@JAndrassy regarding your doubt about the WiFi channel, I don't think it's a problem and in fact as you can see from this screenshot the WiFi manager works as expected.