Sorry, but I also read it from others
and I did it and it was done for me.
When I came back to Android 7.1,
I came back, as Ulefone service says.
https://mega.nz/#F!9zxnDAgb!E13mzplcYDPo8K3XYqbEhw
All she still wanted is to fix the imei, and the "Tee key" does not exist.
Then installing...