- Feb 14, 2017
Since you are not concerned about existing data:
- from TWRP Advanced menu wipe System/Data/Dalvik/Cache partitions
- from TWRP Install menu flash modified FireOS 4.5.5 ROM
That's it. If FireOS can not 'see' the serial number the device many have been black listed by Amazon and/or altered at the hardware level before you acquired it.
Remote possibility the OTA block is creating an issue. You can remove it following the guidance in this and this post. If successful the device will try to download an OTA update but will not be able to apply it as it has dependencies on stock recovery which has been replaced by a custom recovery (TWRP).
So a serial number of zeros indicate a blacklisted kindle? What part has been removed to alter the serial? Then kindle does not appear to have been opened before at all. It looks to be new besides the android os being installed.
I never did an advanced wipe with the initial Fire os flash. I just did the factory wipe is there a difference?