We can determine the billing cycle of the set data warning and data limit on the OnePlus smartphone ourselves, as with some mobile phone providers it is not always automatically the first day of the month.

When the set day is reached, the counted data consumption is set to zero and logged again.

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:  Tap on  Billing cycle
Step 6:  Set the  Billing cycle
Step 7:  Confirm with  OK
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on Data usage
  4. Tap on Billing cycle
  5. Tap on Billing cycle
  6. Set the Billing cycle
  7. Confirm with OK

OnePlus Instructions