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How to install TWRP for the Unihertz Atom L and XL

Getting adb and fastboot

Linux or MacOS

If you are using a Linux or a similar operating system on your PC or Mac chances are very high that you already have these two applications installed. Open a terminal and exectute the following commands.

adb version
fastboot --version

If one or both of these output an error you need to (re-)install adb and fastboot.

wget https://dl.google.com/android/repository/platform-tools-latest-linux.zip
unzip platform-tools-latest-linux.zip -d ~

Update your PATH variable for your environment

gedit ~/.profile

Add the following

# add Android SDK platform tools to path
if [ -d "$HOME/platform-tools" ] ; then
  PATH="$HOME/platform-tools:$PATH"
fi

Then update your environment

source ~/.profile

Windows

Download the latest version of adb and fastboot to a directory of your choice and extract the archive.

Note: I've had reports that under Windows adb/fastboot might not recognize the phone while in TWRP. For now I don't know whats causing this. So if that happends to you try using linux instead.

Unlocking

To be able to install anything on the Unihertz Atom L or XL we first need to unlock the bootloader.

  1. Boot your device into the official OS.
  2. Go to Settings > About phone, tap the "build number" several times to enable developer settings.
  3. Go to Settings > System > Developer Settings, enable OEM unlocking and ADB debugging.
  4. Connect your phone to your PC and open a terminal or a command line window.
  5. Run adb reboot bootloader on your PC (there is no way to enter bootloader directly, only possible through adb).
  6. Once your device has finished booting run fastboot flashing unlock and comfirm unlock on device (THIS WILL WIPE ALL DATA!).
  7. Run fastboot reboot to reboot your device and now you should see an unlocked warning during boot screen.

Disabling AVB (Android Verified Boot)

To get TWRP to boot proberly we also need to disable AVB. Otherwise the bootloader will check the ROM with the wrong authorization keys and prevent the loading.

  1. Download vbmeta.img from the latest release page of your device.
  2. Connect your phone to your PC and open a terminal or a command line window.
  3. Run adb reboot bootloader on your PC to put your device in bootloader mode.
  4. Once your device has finished booting run fastboot --disable-verification --disable-verity flash vbmeta vbmeta.img.
  5. Then run fastboot --disable-verification --disable-verity flash vbmeta_system vbmeta.img.
  6. Also run fastboot --disable-verification --disable-verity flash vbmeta_vendor vbmeta.img.

Installing TWRP recovery

Attention: Make sure you have the correct stock ROM release to the kernel of the TWRP release. If not use the upgrade guide from my lineage tree to switch stock ROM releases.

  1. Download recovery.img from the latest release page of your device.
  2. Connect your phone to your PC and open a terminal or a command line window.
  3. Run adb reboot bootloader on your PC to put your device in bootloader mode.
  4. Once your device has finished booting run fastboot flash recovery recovery.img.
  5. Run fastboot reboot and after the screen goes dark press volume up until you see the TWRP logo.
  6. Now you need to install a new ROM because booting into stock ROM will replace TWRP with the stock recovery.