Solution:
Because Android 6.0 and below are inserted into the USB data cable, the default selection of the mobile phone is the MTP protocol. If both of them are closed by mistake, you can try the following methods:
1 Restart the phone;
2 Try to install the third-party software USBModelSwitch to solve the problem (the software can be guided to the MIUI forum to find).
3 If the above cannot be achieved, it is recommended to restore the factory settings after backing up the data.
Tip:
Android 6.0 and above models do not have the option to switch the MTP protocol, which is enabled by default.
If the connection data cable and other terminals do not have this protocol option, please refer to the article After the mobile phone is connected to the computer via the data cable, what should I do if the computer does not recognize the storage of the mobile phone?