Hi @Raul, not sure whether this is the issue, but might be worth a shot: for me, adb did not find my phone either, i had to add its ID manually first – so maybe this is the same issue for fastboot?
To add the ID (see here for my original post):
find the ID via the “lsusb” command:
- connect the phone
- it might be necessary to put the phone into one of the two USB-modes “Transfer files” or “Transfer photos (PTP)” – I don’t remember
- run lsusb from the linux terminal
- disconnect the phone and run lsusb again –> you see which ID is missing, that was the phone
then:
- add “0x” + the first 4 characters of the ID (before the colon, so, e.g. “0x1234”) and add this string (w/o the quotation marks) to the file ~/.android/adb_usb.ini (create it if it does not exist)
but, again, this might not be it 
– good luck!