These Settings must be properly configured to keep Mobile Tracker Free working.
Info: A password may be required to access the application settings. If you have not changed, the default password is *1234*. More information about the application password here.
1) Settings => Battery => Advanced settings => Optimize battery use => « Wi-Fi » => Don't optimize
2) Settings => Apps => Auto launch => « Wi-Fi » => Turn ON
3) Settings => Apps => App management => « Wi-Fi » => Data usage => Background data => Turn ON
4) Settings => Apps => App management => « Wi-Fi » => Permissions => Location => Allow all the time