WLED AP not showing up

Hello everybody,

I bought a couple of LS4P controllers, but I am struggling with setting up the WLED. I am powering the controller with a 5V external power supply and without a light strip attached. There seeems to be voltage going in the controller (check it with a multimeter), but the WLED AP is not showing. I tried holding the button down for a number of seconds but it did not work either. I tried with another controller, thinking that I could have messed up the wiring (which I did not), but the result was the same - no AP showing. Could you give me a guide on how to proceed? I figured out I might have to manually flash the controller, but I am clueless on how to do that.

Thanks in advance!