The network mode or preferred network type is set by default to automatically search for 2G/3G/4G to achieve the best possible data connection and voice quality.

For example, if we have poor to moderate 4G reception area by area with the OnePlus smartphone, we can set the network mode ourselves to use the full width of the 3G network.

Note: If you experience reception problems during calls or phone calls, you can try setting the network mode to 2G to see if the problem is with network coverage.

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:  Tap on  Preferred network type
Step 5:  Choose  2G/3G/4G (Automatic) ,  2G/3G (Automatic)  or  2G only
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on SIM & network
  4. Tap on Preferred network type
  5. Choose 2G/3G/4G (Automatic), 2G/3G (Automatic) or 2G only

OnePlus Instructions