One way to fix the null IMEI issue on your Xiaomi Mi 9T smartphone is by using an XQCN file. Here are the steps to follow:

First, download the XQCN file for your Xiaomi Mi 9T model.
Next, download and install a Qualcomm USB driver on your computer.
Connect your Xiaomi Mi 9T smartphone to your computer using a USB cable.
Open a command prompt window on your computer and enter the following command: "adb devices". This will show if your device is connected.
Once your device is detected, enter the following command: "adb shell".
Then, type "su" to grant superuser permission.
Enter the following command to backup your current QCN file: dd if=/dev/block/bootdevice/by-name/modemst1 of=/sdcard/modemst1-backup.qcn
Now, flash the XQCN file you downloaded earlier using this command: dd if=/sdcard/your-filename.xqcn of=/dev/block/bootdevice/by-name/modemst1
Restart your phone and check if the IMEI number has been restored.

Please note that flashing XQCN files can be risky and may cause issues with your device if done incorrectly. Make sure to follow the instructions carefully and proceed at your own risk.

You can also use UMT (Ultimate Multi Tool) to fix the null IMEI issue on your Xiaomi Mi 9T smartphone. Here's how:

Download and install the Ultimate Multi Tool (UMT) on your computer.
Launch the UMT tool and select "Qualcomm" from the "Brand" drop-down menu.
Click on the "IMEI" tab and select "Write Dual IMEI" option.
Enter the IMEI numbers you want to write in the "IMEI1" and "IMEI2" fields.
Connect your Xiaomi Mi 9T smartphone to your computer using a USB cable.
Power off your device and boot it into EDL (Emergency Download) mode by pressing and holding the Volume Up + Volume Down keys together while connecting the USB cable to your device.
Once your device is detected, click on the "Start" button to begin writing the IMEI numbers to your device.
Wait for the process to complete and then disconnect your device from the computer.
Power on your device and check if the IMEI number has been restored.

Please note that this process requires technical knowledge and should be done carefully. Ensure that you have backed up your device data before proceeding. Additionally, changing the IMEI number of your device may be illegal in some regions, so please make sure to follow the regulations in your area.