Xiaomi
Turn Data Usage On or Off for a SIM Card auf Xiaomi
When a SIM card stops using mobile data on a Xiaomi phone, the Data usage switch inside the Security app is almost always responsible. MIUI ties this switch directly to the data limit feature: once your phone reaches the threshold you set, the system flips it off automatically to stop further charges.
You find the control at Security ➔ Settings ➔ Data usage. From there you can enable or disable Data usage for the SIM card manually, independent of whether a limit has been reached yet.
Turning the switch back on restores mobile data immediately, but if a limit is still active, the connection can shut off again once you cross it during the current cycle. If your phone has no active data limit and mobile data still will not connect, check the individual app permissions under mobile data or the network mode of the SIM instead - this toggle only controls the general on/off state, not per-app access or signal issues.
Disabling Data usage here does not affect Wi-Fi and does not delete your usage statistics. It only blocks the SIM from sending or receiving data over the mobile network until you switch it on again.