Xiaomi
Set the Action for When Your Data Plan Runs Out auf Xiaomi
MIUI tracks your mobile data usage against a plan you define yourself, not against your carrier's billing cycle. Once that self-set limit is reached, the system needs to know what to do next: keep the connection running with just a notification, or cut mobile data automatically to prevent overage charges.
This behavior is controlled in the Security app, not in the general connectivity settings where most people look first. That is why the option is easy to miss, especially since the default action can silently shut off data without an obvious warning beforehand.
Where this setting lives
Go to Security ➤ Settings ➤ Data usage ➤ Data plan ➤ When data plan is used up. From there, choose either Show warning to keep mobile data active with a notification, or Warn and turn off mobile data to have MIUI disable mobile data automatically once the limit is hit.
If you only want a heads-up without any risk of losing connectivity, Show warning is the safer pick. If avoiding overage charges matters more than staying connected, the auto-shutoff option is the better fit.
Keep in mind this setting only works if you already entered a realistic data plan value under Data plan. An unset or inaccurate limit means the warning or shutoff never triggers at the right time.