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 => Battery optimization => « Wi-Fi » => Don't optimize
2) Settings => Apps & notifications => See all xx apps => « Wi-Fi » => Mobile data & Wi-Fi => Backgroud Data & Unrestriced data usage => Turn ON