The SMS delivery report notifies us when the SMS has reached the recipient. On Oppo smartphones, the SMS delivery report is deactivated by default, but can be activated in the message settings, even separately for each SIM card.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Messages -App
Step 2:  Open the  Menu
Step 3:  Tap on  Settings
Step 4:  Tap on  More settings
Step 5:  Tap on  Fetch SMS delivery reports
Step 6:  Enable or Disable  SIM1  /  SIM2
  1. Open the Messages-App
  2. Open the Menu
  3. Tap on Settings
  4. Tap on More settings
  5. Tap on Fetch SMS delivery reports
  6. Enable or Disable SIM1 SIM2

Oppo Instructions