Samsung
Turn Data Usage Limit On or Off auf Samsung
The Samsung data usage limit works as an automatic cutoff, not just a warning. Once your tracked usage reaches the value you set, Samsung One UI disables mobile data completely for the rest of the billing cycle, which by default resets on the 1st of the month.
This function lives inside the Data usage menu, separate from the general mobile data toggle. If you only want a heads-up before reaching a cap, the data warning threshold in the same menu fits better, since it notifies you without cutting the connection.
To enable or disable the limit itself:
Tap on Settings ➔ Tap on Connections ➔ Tap on Data usage ➔ Tap on Billing cycle and data warning ➔ Enable or disable Set data limit
On the same screen you can adjust the billing cycle start date and the exact limit value. Keep in mind that data counted on the device can differ slightly from your carrier bill, since background syncs and system processes are measured differently on the phone than on the network side. If you rely on precise billing figures, compare the value against your carrier app before locking in a strict limit.