There are two ways to root that device that worked for me so far. One of them is this one and have always worked flawlessly. It needs, as a prerequisite, USB debugging to be toggled on :
- download payload dumper on your computer
- extract the payload.bin from the archive of the rom you're currently on
- put the payload in the folder named payload input
- run the payload application and wait until you see "boot" on the command prompt at least
- go to the payload output folder, copy the boot.img and paste it on your phone sdcard
- open Magisk Manager, press Install > Patch a file with Magisk, and select your boot.img
- copy and paste the patched file (which name, by default, is magisk_patched.img) in a folder of your computer
- press shift and right click in this folder and open a command prompt
- type "adb devices" just to make sure your device is recognized, if it is, type "adb reboot fastboot"
- once you're in fastboot mode, type "fastboot devices" to make sure that your phone is recognized and, if it is, type "fastboot boot magisk_patched.img" (you can skip that step but I don't recommend to do so as it enables you to make sure that your device will boot properly after flashing the patched boot.img)
- if you didn't skip the previous step and it worked, reboot into fastboot mode, make sure that your device is recognized again and then type "fastboot flash boot magisk_patched.img"
- wait until flashing is done and then type "fastboot reboot"
You should be rooted properly at this point. Please excuse my English, as it's not my native language, I surely made some mistakes !
Have a good day and don't hesitate to tell me if this method worked for you !