For those who may also have to deal with this problem, here are the steps taken

Just use Fastactivate or mine A_F_Activate, both are using that tool.
Connect to your pc so you're able to see your device as an extra drive
Copy the Navcore to your device
Reboot the divice

I didn't test it, but already some other did succesfully
When your device will start
Copy than bootloader 0.0001to your device the name is system
Boot again
When it start copy the next bootloader 5.5128 to your system and boot your system again.
If all went into the right way you may install any other Navcore which you like

But mind you, check if the Navcore has a file in it with the name system, remove it at once
When the version of that system file is higher than 5.5128 it will be installed as an upgrade, do this only when you want it

To downgrade follow the same steps and start again with bootloader 0.0001 and so on