@jock2 I tried multiboot (latest version) many times and with repeated power cyclings but no success.
When I have time I will solder the 3 pins for jtag and make the serial debugging.
BTW, should I set 5V or 3,3V as TTL level ?
3,3v should be fine, but it's not a jtag but a simple serial port.
Be sure to burn the image on a perfectly working sd card with a proper sd burner like balena etcher, which is able decompress and verify written sectors automatically.
If a led is blinking it means that the kernel is alive and you should see something on the HDMI.
If the leds are turned off or dimmed it means something is wrong during boot.
If the original firmware starts, either the box has a very old firmware or the sdcard is somehow faulty