I cannot get the results for either the 5 s hold-down or the 30 s hold-down of the WLAN RST button.
What can I do ?
That's not surprising, don't bother trying. For the five and thirty second resets to work, you need to wait for the Linux system to be fully booted before starting the button press. Your system is not booting; it's failing very early in the process. The code that looks for the button press is a long way away from running. As you found, those resets aren't going to help you. I will let one of the Linux experts reply and give more details, but I'm thinking you may have to manually reflash the system: Reflashing the OpenWrt-Yun image on the Yún