Edit:
After unlocking it I was not able to install a custom recovery, because the phone would reboot in "Fastboot & Rescue Mode" and my computers (windows and linux) would not see it anymore, so unable to download anything. I tried to flash a stock firmware again trough vol+ vol- power , but now the phone is locked again and the fastboot and rescue mode does not allow me to flash anything simply because the computers don't see it, even if the phone boots up and works fine (and the computers see it).
Any suggestions ? Thanks