Silent mode can be activated or deactivated both via the volume buttons and via the settings themselves.

For the volume buttons, the volume must be reduced until it is in vibration mode. Pressing the Volume down button again will activate the Silent mode. The mode can be deactivated in the opposite way.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  Sound
Step 3:  Tap on  Sound profile
Step 4:  Choose between  Silent  and  Sound
  1. Open the Settings
  2. Tap on Sound
  3. Tap on Sound profile
  4. Choose between Silent and Sound

LG Instructions