In addition to calls, a standard SIM card for mobile data can also be changed and the device will only use the specified SIM card for mobile data.

Note: After changing the default SIM card, the device will search for a network again.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  SIM card & mobile data
Step 3:  Tap on  Mobile data
Step 4:  Choose between  SIM1  and  SIM2
Step 5:  Confirm with  Switch
  1. Open the Settings
  2. Tap on SIM card & mobile data
  3. Tap on Mobile data
  4. Choose between SIM1 and SIM2
  5. Confirm with Switch

Oppo Instructions