P600 in retail demo mode.

[stantonworrier]

Hi all
I am relative noob ( my first post) so I recently bought a p600 from a well known internet auction site to experiment with. I knew that it was an ex demo model and was in retail demo mode.

I have tried to push TWRP using adb sideload (ADB connects to the device) and also tried using Heimdall ( recommended on the lineage OS "howto") however the util reports no device in download mode.

Please could someone inform if I am wasting my time? Is the device is permanently "locked" in demo mode?

In recovery mode when I try to ADB sideload I see:

"Android system recovery <3e>
reboot system now
apply update from ADB
Wipe data/factory reset
wipe cache partition
apply update from cache"

When I tried to push a TWRPrecovery image from ADB it fails. When I tried to push a TWRP.zip using ADB sideload it also fails.
" #MANUAL MODE#
--Applied the CSC-code : PAP
Successfully applied multi-CSC

Now send the package...
E:failed to verify whole-file signature
E: signature verification failed.
Installation aborted"

Please could someone point me at a solution?
TIA

Stanton