We can activate a data alert on the OnePlus smartphone to gain a little more control over the consumption of mobile data and, if necessary, limit ourselves when the data volume is used up.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  Wi-Fi & network
Step 3:  Tap on  Data usage
Step 4:  Tap on  Billing cycle
Step 5:  Enable or disable  Set data warning
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on Data usage
  4. Tap on Billing cycle
  5. Enable or disable Set data warning

OnePlus Instructions