In order to use a new keyboard, it must first be activated in the Keyboard Manager, only then can it be selected under Standard Keyboard.

Note: This guide only shows how to enable or disable a keyboard, to use it you have to change the default keyboard.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  General
Step 3:  Tap on  Language & keyboard
Step 4:  Tap on  On-screen keyboard
Step 5:  Tap on  Manage keyboards
Step 6:  Enable or Disable desired  Keyboard
Step 7:  Confirm with  OK
  1. Open the Settings
  2. Tap on General
  3. Tap on Language & keyboard
  4. Tap on On-screen keyboard
  5. Tap on Manage keyboards
  6. Enable or Disable desired Keyboard
  7. Confirm with OK

LG Instructions