Solution:
Open a clap on Mi Robot Vacuum-Mop 2 front cover.
Use a paper clip or something similarly shaped to press Mi Robot Vacuum-Mop 2 reset button.
Keep it pressed for over 3 seconds.
When Mi Robot Vacuum-Mop 2 Wi-Fi LED indicator blinks and you hear a voice message "Restoring factory settings" you can release Xiaomi Vacuum Reset Key.
Wait a moment until Mi Robot Vacuum-Mop 2 reboots, and is ready to be reconnected with the Xiaomi Home app.