DavidPrez Hello,
You failed to flash using SPFlashtool because you are using an incorrect connection.
I have checked your thread, the two flashing tools you are using cannot recognize your device connection.
You may try connecting your device in BROM mode instead of fastboot mode, by simultaneously pressing the Vol Up + Vol down buttons and inserting the USB cable.
If the above method still doesn't force your device to brom mode, you may have to find a testpoint on the PCB board of your device.