MR33 and OpenWRT installation problems

Hi,
I have two MR33s with Firmware MR 30.7.1, and after several attempts, I can't install any version of OpenWRT.
After connecting the device's serial port to the PC's USB port and powering the MR33 board, virtually nothing happens. It's still asking to turn on the device.
The serial port is connected correctly because with Minicom I can see all the boot phases.
I followed the steps exactly as reported on the official page:

Do you have any advice for me?
Thanks in advance.
Davide

can you record console output of your serial port activities? Hard to guess what went wrong.