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) i Manager 
=> Permission management => Apps => « Wi-Fi » => Single permission settings => Turn ON All
2) i Manager => Data Monitor => Network management => « Wi-Fi » => Data network & WLAN => Turn ON
3) i Manager => Virus scan => Settings icon => Regular scan reminder => Turn OFF
4) Settings 
=> Mobile network => « Wi-Fi » => Turn ON
5) Setting => Mobile network => Data-saving mode => Whitelist apps => Wi-Fi => Turn ON
6) Settings => Battery => High background power consumption => Wi-Fi => Turn ON
7) Setting => Status bar and notification => Manage notification => Whatsapp/Messenger/Other social apps => Allow notification => Turn ON
Example for WhatsApp
Hai