Your unused data amount from the previous month will be rolled over to the next month.
Here is the graph illustrating the rollover system for a mobile plan:
e.g. If you have the 25 GB SIM plan and used 20 GB this month (5 GB remaining at the end of this month), you will have 30 GB (25 GB + 5 GB) total available data allowance next month.
*The rollover data amount is valid for the next 1 month. Therefore, if you have 5 GB remaining data from this month, you can only use it until the end of next month.