Android
Set a Monthly Data Usage Limit auf Android
Android tracks mobile data usage per SIM, but it only warns you by default. A warning does not stop data flow, it just shows a notification once a threshold is crossed.
A data limit works differently. Once the set amount is reached, Android disables mobile data automatically for that SIM. This prevents extra charges but also stops any app that relies on mobile data, including background syncs, calls over data, and downloads.
To set it, open Tap on Settings ➔ Tap on Network & internet ➔ Tap on SIMs. On dual SIM phones, pick the SIM the limit should apply to before continuing. Then go to Tap on Data warning & limit ➔ Tap on Data limit, enter the amount that matches your plan, and confirm with Set a Data usage limit and tap on Set.
Set the limit slightly below your actual plan allowance to leave a buffer for usage that Android reports with a short delay. If you only want a heads-up without cutting the connection, use the data warning option instead and skip the hard limit.