By default the Oppo-Smartphone automatically searches for a network, but once the Automatic search has been deactivated to manually select the network, it can be reactivated 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:  Tap on  Carrier
Step 5:  Enable  Auto select
  1. Open the Settings
  2. Tap on SIM card & mobile data
  3. Choose desired SIM card
  4. Tap on Carrier
  5. Enable Auto select

Oppo Instructions