Here are detailed steps for Samsung Android 9.x to keep FamiGuard Pro app working.
Step 1: Settings 
> Device care > Battery > Menu > Settings
- Notifications > Turn OFF
- Adaptive battery > Turn OFF
- Put unused apps to sleep > Turn OFF
- Auto disable unused apps > Turn OFF
- Sleeping apps > Check that the application is not in this listed.
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Optimize settings > Turn OFF
Step 2: Settings > Device care > Memory > Menu > Apps that aren't checked > Add > WhatsApp Service > Add
Step 3: Settings > Biometrics and security > Permission usage monitor > WhatsApp Service > Turn OFF
Step 4: Settings > Apps > WhatsApp Service > Mobile data > Allow background data usage & Allow app while Data saver on > Turn ON
