kindle fire hdx 3rd generation can't twrp

Hi

Newbie here. We bought a kindle which turnout to be blacklisted so we can't register it. Obviously it is useless as we can only use internet.
My kindle was running on latest firmware which meant I couldn't root it. Thanks to one of you xda geniuses. I was able to rollback to firmware 13.3.2.8
I change oat filed to avoid upgrades. I installed es explorer.
I then rooted my kindle and did root checked which confirms it is rooted on Kfthwi android 4.2.2
I installed su super user. Seems to work fine
After that I tried to install boot loader fireflash but it keeps stopping
I finally managed to install and launch Rashr but when I boot recovery with twrp or any other files my kindle boots and stops saying can't boot and offers me to do factory reset or starts again as if nothing happened. I haven't chosen factory reset as I don't think this will help.
I have added su super users to system hoping this would help but no success.

I have also tried loads of kfu from pc but although adb is ok it says boot status unknown and fails every time as if it can't find the path or something...

I have ran out of ideas. I don't know what to do anymore

My aim is to install cm12 or other versions... anything to make this kindle become android
Please help me!
Thanks