It finally worked!
The truck was simply trying another PC! It seems that the main PC I used had the USB drivers messed up due to the various trials.
I used the flashing software provided in the alldocube page, the latest December firmware. The only difference from the normal flashing process was plugging in the battery pins while pressing the VOLUME Down button.
Now I have the latest firmware version too. SIM is working fine too.
Thanks for the help and convincing me to keep trying!!