By default, Oppo-Smartphone automatically searches for a network. The criteria here are not only the home network, but also the quality of the network. In some cases, for example at the border or abroad, it makes sense to select the network manually.

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:  Disable  Auto select
Step 6:  Tap on  Off
Step 7:  Confirm with  Off
Step 8:  Choose desired  Network
  1. Open the Settings
  2. Tap on SIM card & mobile data
  3. Choose desired SIM card
  4. Tap on Carrier
  5. Disable Auto select
  6. Tap on Off
  7. Confirm with Off
  8. Choose desired Network

Oppo Instructions