Thank's to yasasbandara :
https://forum.xda-developers.com/member.php?u=4672130
I would like to quote his very good article from here:
https://forum.xda-developers.com/showpost.php?p=76098771&postcount=143
English Guide for TWRP+Root+Custom Rom
Step 1 – Preparation
Step 2 – Installing Chinese Rom (to get an Unlocked Bootloader) via Stock Recovery
Step 3 – Using ADB to replace Stock Recovery with TWRP
Step 4 – Installing Custom/Official Rom/Roms and Rooting
Step 1:-
Prepare a SD card and copy the china rom (NX551-update_1.59.zip), Super SU Zip (SuperSU-v2.82-201705271822.zip) and any Custom Rom you wanna flash.
Get rid of any encryptions, fingerprints, passwords etc. you may have put on the phone.
Copy contacts, sms, call logs etc. (use super backup) and store on SD card.
**You can use a tool called "Nubia Emergency Tool" for backing up the service areas of the phone (optional). This backup will contain IMEI numbers and stuff. It'll help you later if you turn up with a brick after a bad custom rom flash**
Charge fully.
Download a flasher package like GizFlasher M2 v2 etc. prepared by developers in the gizchina forum, Greek Forum or 4PDA (to search for these, just use google and type nubia m2 gizchina/nubia m2 greek forum etc. Use google translation for understanding).
Step 2:-
Turn the phone off. Press Power & Volume up --> Boot to Stock Recovery. Use wipe function to wipe caches and data. Use update function and select the NX551-update_1.59.zip file and flash it. This will give you an unlocked bootloader.
Step 3:-
Boot to system. Select language and stuff. Turn on developer options. Tick OEM unlocking and USB debugging.
Use GizFlasher M2 v2 (or similar packages from other developers) to install ADB. For GizFlasher M2 v2, uncompress into a folder and run ‘Autorun.exe’ with admin rights. Answer ‘Yes’ to all and install ADB drivers systemwide.
You can check drivers by connecting the phone in “charge only” mode. Check device manager to see if ‘Android ADB interface’ is available. Give permission in the phone if required.
(If you find unknown Android devices in device manager, chose driver update (right click) --> Browse drivers on my computer --> Choose from list of drivers on the computer --> Android device --> Next --> Android ADB interface version.)
If the drivers are OK, keep the phone in charging only mode and check adb installation with a command like “adb devices” using the command prompt. This should initiate adb service and drivers (useful in Win10). Your device name will be displayed. Close the command prompt.
In the GizFlasher folder, run the "GizFlasher.bat" with administrator rights. In the window that appear, give "1" and enter for unlocking the bootloader (confirm on the phone if required).
Wait for the phone to restart.
The phone will restart in fastboot mode.
After this, type "2" and enter for flashing TWRP.
TWRP will be flashed and the phone will start in the new Recovery. You can unplug the phone from the PC.
*** Immediately after the TWRP, it’s always a good idea to setup language, time and brightness and remove screen timeout. Do a TWRP backup using the SD card.***
Step 4:-
Please note that using the stock recovery and ADB commands are usually harmless and rarely cause issues. But from this point onwards, it’s always a good idea to get a TWRP backup stored in somewhere safe (in your PC). Furthermore, if you wanna be more careful, take a backup of the service areas of your phone using a program called “Nubia Emergency Tool”. You’ll need the latest Chinese only version and instructions will be available at the 4pda forum.
Once, you’ve chosen a custom rom, please read instructions by the respective developer and follow those instructions.
Boot to TWRP recovery and do the reset. As a general rule, always wipe caches and data (full reset) and flash the rom. You’ll need to flash the SuperSU zip afterwards and wipe caches to obtain root.
If the ROM is working properly, restore your contacts, SMS etc. using super backup.
Note -
** Older phones that did not come with a higher 'Global Rom' version have their bootloaders already unlocked. In such a case, you can skip the Chinese Rom installation.
** For those who are having problems with ADB and Fastboot in newer Windows 10 versions - This is not your fault. The problem is probably with Windows. You can try to force-remove driver signature checking [restart with shift key-->troubleshooting-->Advanced-->restart-->remove sig. checking (option 7)] and do everything over.
However, do not waste a lot of time going through the same dance over and over again. The easiest way to use ADB/Fastboot is via Linux. That's an undisputed truth.
Just search for a project called FWUL (F*** Windows Use Linux) in XDA forum and follow instructions. You'll need a small pen drive (2GB or more) and that's it.
