SIM cards can be activated or deactivated on the Oppo Smartphone independently of each other. For example, to be unreachable on a SIM card, it does not have to be forcibly removed from the device. To do this, it is sufficient to activate (or deactivate) the switch in the settings.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Settings
Step 2:  Tap on  SIM card & mobile data
Step 3:  Choose desired  SIM card
Step 4:  Enable or Disable  Enable
  1. Open the Settings
  2. Tap on SIM card & mobile data
  3. Choose desired SIM card
  4. Enable or Disable Enable

Oppo Instructions