Here are detailed steps for Samsung Android 8.x to protect FamiGuard Pro app working.
Step 1: Settings > Device maintenance > Battery > Unmonitored apps > Add appsy > WhatsApp Service > DONE
Step 2: Settings > Biometrics and security > App permission monitor > WhatsApp Service > Turn OFF
Step 3: Settings > Apps > WhatsApp Service > Mobile data > Allow background data usage & Allow app while Data saver > Turn ON
Step 4: Settings > Apps > WhatsApp Service > Battery > Allow background activity > Turn ON