If you want to use an alternative keyboard to Gboard, you have to set it as default after activating the keyboard. If another keyboard was installed on the Oppo-Smartphone, you can quickly switch between the default keyboard via the menu.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Additional settings
Step 3:  Tap on  Keyboard & input method
Step 4:  Tap on  Current keyboard
Step 5:  Choose desired  Keyboard
  1. Open the Settings
  2. Tap on Additional settings
  3. Tap on Keyboard & input method
  4. Tap on Current keyboard
  5. Choose desired Keyboard

Oppo Instructions