Thanks for the tip.
I had seen a lot of way to reset the RC after an upgrade failure, most using a more complicated selection of buttons, but this is the first one that bring some change in the status lamps and show it is not totally bricked.
The combination of C1 / shutter plus power button seems to at least turn on/off the unit. Now I can see on battery LEDs its level and charge progress. It is detected by a tablet (tried android and ios) but the upgrade fails at 0% after the status LED becomes blue and start beeping. Then it turn just off and is back to the original state.
At least it is a big step ahead and hope remains.
EDIT: My status LED color is somehow between green and blue and the RC is GL300C, no way to use USB stick to upgrade the firmware. Time to read the firmware upgrade manual for alternatives.
I made an account just to thank you... Thank you sir!