!!!!! This is the specific step: Accessibility–Touch–Assistive Touch–Devices
If your iPad/iPhone is not ios13 or ios13.1, you need to do the following steps:
1. Your device has been “Jailbreak”, enter Cydia to search for BTstack Mouse and install this software.
2. After the installation is complete, find the Mouse icon on the iPad desktop and click to enter.
3. Go to “Mouse”, open the Bluetooth-compatible mouse, it will automatically search (press the Bluetooth-compatible pairing button at the bottom of the mouse)