To mark a message as read it is sufficient to open the conversation; if several messages (SMS) have been received in different conversations, all messages can be marked as read via the menu.

Android 9 | LG UX 9.0
Step 1:  Open the  Messages -App
Step 2:  Open the  Menu
Step 3:  Tap on  Mark all as read
  1. Open the Messages-App
  2. Open the Menu
  3. Tap on Mark all as read

LG Instructions