Kindle j9g29r (10.gen) Demo Unlock ?

[MaTaD]

Hello, first of all I have to apologize because English is not my greatest strength.

So I'll try to describe my problem a little more simply.

I have a Kindle model no. j9g29r that hangs in demo mode. I know my way around a bit now and have first looked in which version it hangs.

He goes into the "I'm only showing 6 PDF" right after the boot.
So I opened the Kindle and connected the serial connection.

Here I could neither see the root login nor in the backup menu Set E for Fat32 or U for Update. Unfortunately, option 3 doesn't do anything.

What I can go into fastboot where it displays "Press Ctrl+C for Exit"
But unfortunately I have no idea what to do with it anymore.

Is there a guide or can you unlock it at all at the moment.

Thanks in advance for your help.