If Automatic Disconnection of Network Connections is enabled on the OnePlus smartphone, we can set our own data limit.
Once the set value is reached or exceeded, all mobile data connections are disconnected and will not be reactivated until the selected billing cycle is reset.

A network connection via a WiFi network is still possible without restrictions.

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  Data limit
Step 6:  Set a  data limit  and tap  OK
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on Data usage
  4. Tap on Billing cycle
  5. Tap on Data limit
  6. Set a data limit and tap OK

OnePlus Instructions