@Ad As @domubpkm stated, only fastboot is needed. Fastboot is also the boot loader, so boot to fastboot, open the UT installer and manually select your device from the drop down list on the installer front page. Connect your device and set the installer going, following the instructions and NOT leaving the wipe data bo unticked.

Good evening Iduboeuf, Thanks a lot for your answer and suggestions! And sorry to reply only only. I will check again the APN, but as far as I remember it was correct. See you soon for the feed back. Ad.