@advocatux ok, I think we are done. In the previous OS version I had on my phone

android-gadget-service enable rndis

was enough. Reading the guide you posted I found out there is a

sudo tethering enable

to be executed. I tried and now this post is flowing through my BQ E4.5
Now I have just two more questions: before to see the command I had already enabled RW permissions on my device. Is there a way to go back to RO without reinstalling the OS or is it impossible as described here? Should I be scared of having a RW OS or is it ok?
Thank you for the support up to here.

@ziggutas be sure you don't have other active connections than USB connection with your phone (ethernet cable, wifi, other USB modem...), otherwise hostname command may return informations about that.
@Lakotaubp I agree, I'm going to create a new thread

@kugiigi already added in the bug report 6 minutes before your last post