OnePlus
Change the Billing Cycle Date auf OnePlus
OxygenOS tracks mobile data against a billing cycle stored locally on the phone, separate from whatever cycle your carrier actually bills on. By default this often starts on the 1st of the month, but most contracts run on a different date tied to when you signed up.
If the two dates do not match, the data warning and data limit trigger too early or too late, and the counted usage resets on a day that has nothing to do with your invoice. This makes the on-screen numbers unreliable for tracking how close you are to your actual cap.
Where to set it
Open Settings ➔ Wi-Fi & network ➔ Data usage ➔ Billing cycle ➔ Billing cycle, then set the correct day and confirm with OK.
Once the date matches your provider, the usage counter resets on the same day your allowance renews. This is only a local tracking setting: it does not change your contract or actual billed data, and it will not fix a limit that is simply set too low for your plan.