If the automatic network selection on the OnePlus smartphone has been deactivated, for example because we were in another country, we can reactivate it at any time.
In this case, the device dials or registers itself in the network of our standard provider.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  Wi-Fi & network
Step 3:  Tap on  SIM & network
Step 4:  Choose a  SIM card
Step 5:  Tap on  Network operators
Step 6:  Confirm with  OK
Step 7:  Enable  Choose automatically
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on SIM & network
  4. Choose a SIM card
  5. Tap on Network operators
  6. Confirm with OK
  7. Enable Choose automatically

OnePlus Instructions