These Settings must be properly configured to keep Mobile Tracker Free working.
1) Settings => Device maintenance => Battery => Unmonitored apps => Add apps => « Wi-Fi » => DONE
2) Settings => Lock screen and security => App permission monitor => « Wi-Fi » => Turn OFF
3) Settings => Apps => « Wi-Fi » => Mobile data => Allow background data usage & Allow app while Data saver => Turn ON
4) Settings => Apps => « Wi-Fi » => Battery => Allow background activity => Turn ON