LG devices without an OLED display do not have an always-on display but a screensaver, which illuminates the entire screen, which means that the screensaver only works when the LG device is connected to a charger cable or docking station.

Optionally, the screen saver can be started when at the charger, docking station or either of them.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  Display
Step 3:  Tap on  Screen saver
Step 4:  Tap on  Automatically start
Step 5:  Choose between  When docked ,  When charging  and  When docked or charging
  1. Open the Settings
  2. Tap on Display
  3. Tap on Screen saver
  4. Tap on Automatically start
  5. Choose between When dockedWhen charging and When docked or charging

LG Instructions