The rugged Doogee S90 is my main phone in the past.

Prerequisites

1. Android SDK Platform-Tools
2. Google USB Driver
3. SP Flash Tool
4. Magisk

Rooting

1. Update the phone first. Make sure the build number is DOOGEE_S90_Android8.1-20191205.
2. Unlock the bootloader. (Backup your files, this will erase all of your data.)
   1. Enable USB debugging and OEM unlocking in Developer options.
   2. Connect phone to your computer.
   3. adb devices to allow ADB access.
   4. adb reboot bootloader to reboot to bootloader.
   5. Type fastboot devices to verify if the phone is connected.
   6. Type fastboot oem unlock to unlock bootloader. (This will factory reset your device.)
   7. Type fastboot reboot to reboot device.
3. Install Magisk.apk on your phone.
4. Download boot.img here. (Or readback boot.img using SP Flash Tool. (Advanced))
5. Patch boot.img using Magisk.
6. Flash patched boot.img using SP Flash Tool.
   1. Download scatter-loading file here. (Or create your own scatter file using MTK Droid Tool. (Advanced))
   2. Select boot.img in SP Flash Tool.
   3. Turn off your phone.
   4. Click Download.
   5. Connect your phone to your PC using a USB cable.
   6. If download ok shows, disconnect your phone.
7. Open Magisk to verify if Magisk has been installed.

Backup

To prevent problems later on, it is recommended to readback important partitions in your phone to prevent a brick if something goes wrong.