Incoming SMS messages are displayed on the LG smartphone by default using the Messages app, but you can also manage your messages using other apps such as the popular Facebook Messenger app. To receive messages (SMS) from another app, you need to set that app as the default app.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  General
Step 3:  Tap on  Apps & notifications
Step 4:  Tap on  Default apps
Step 5:  Tap on  Message app
Step 6:  Choose desired  Message app
Step 7:  Confirm with  Change
  1. Open the Settings
  2. Tap on General
  3. Tap on Apps & notifications
  4. Tap on Default apps
  5. Tap on Message app
  6. Choose desired Message app
  7. Confirm with Change

LG Instructions