I bought a Xiaomi Note 9 Pro Global, MIUI 12 with Android 10 and I have installed Ubuntu Touch on it. Android 10 must be installed because it is built on Halium 10.

The price of this phone was 240 € brand new. I got it from Mediamarkt, Madrid, Spain. It is still available new in some places, as it is a high-end device. It can be bought second hand for between 150 and 200 €.

Before inserting a SIM card or connect it to a wifi network, I blocked the updates, so that the phone would not update.

To unlock the bootloader, create a Xiaomi account from the phone, you can add your current Google account. You must add the phone number associate to the device.

Download the Mi Unlock App here: https://c.mi.com/thread-2262302-1-0.html available for Windows 10. Run it and login with the account create above and connect the phone to the computer. Once started, you must wait about 155 hours to unlock the device.

If the phone doesn’t come with Android 10, a new rom must be installed. Look here and download the version for your phone: https://github.com/ubuntu-touch-miatoll/ubuntu-touch-miatoll/blob/main/FIRMWARE.md
The following link is valid one and works WAYDROID :

Redmi Note 9 Pro Global EEA
Stable
Fastboot
V12.0.1.0.QJZEUXM
Android 10.0
4.3 GB
2020-11-10
Download https://xiaomifirmwareupdater.com/miui/joyeuse/stable/V12.0.1.0.QJZEUXM/

You also need the Xiaomi program to flash your phone , download for Windows 10 MIUI ROM Flashing Tool https://c.mi.com/oc/miuidownload/detail?guide=2
when flashing the phone see if it has the option to not lock the bootloader, it must be unlocked.

The rest of the process, I did them on Ubuntu 20.04.

Install ADB and FASTBOOT on the PC via the terminal typing:
sudo apt install android-tools-adb android-tools-fastboot
Turn on the phone with Android 10 installed, and enable Developer mode and USB Debugging.

Connect the phone to the computer with a cable in a USB 2.0 port. USB 3.0 ports doesn’t work.

Open a terminal and type:
adb devices
You’ll see something like:

$ adb devices
List of devices attached
* daemon not running; starting now at tcp:5037
* daemon started successfully
8xxxxxxx	unauthorized

A pop-up window will appears on the phone screen. Tap on accept to authorize USB Debugging.
Then, on the terminal type:
adb devices
You will see something like:

$ adb devices
List of devices attached
8xxxxxxx	device

This shows the id of your mobile phone and “device”. This means that it is properly connected.

Next, install the UBports Installer 0.9.5 on your computer. It is available as an appimage, .deb and in snap packeges.

Start the UBports Installer. It should detect the mobile model if it connected to the computer and turned on.
Check that the mobile model is correct and set the channel to Stable, 16.04 and enable both, the
“Wipe data” and “Bootstrap” options.

Press Install to start downloading the files needed to the computer.
Then, the installer will try to reboot the device in fastboot mode. If this fails, you’ll have to do it manually: Turn the phone off and then turn it on pressing volume down and power buttons at the same time. Don’t unplug the phone from the computer for the installer to continue with the installation.

Once a purple screen with the name Ubuntu Touch appears on the mobile screen, the OS is installed. Congratulations.

Xiaomi Note 9 Pro porting
BackMarket

This phone works very well, I use it daily with Ubuntu Touch,
To get the phone to recognise both SIMs, first insert one SIM and make calls, then turn the phone off and you can insert both SIM cards, now both SIMs work.

It has some issues,

Occasionally the sound drops in volume when using various apps that use sound or the microphone stops working with the recorder app.
The front camera does not work in video calls.
If you use Waydroid the GPS does not work, you must restart UT to get it working again on UT.

Regards...