How to use full data allowance without incurring extra charges?

How to use full data allowance without incurring extra charges?
How can you use all of your data allowance and run out before the end of the month without incurring any charges for over use?
What happens when you use the last of your data?
I have found it difficult to monitor data use in my account, with large amounts of data use being added at odd times.
This means as you get to the last, say 10gb of your data you have to turn it off in case you get any excess data charges. Or does data just stop till the first day of the next month?
Here is my recent data usage

There is a 0 spend cap on lebara by default unless you turned on auto top up.
If you use all data the connection will stop.

My Auto Top Up is turned off so I can use my data to supplement my limited home broadband sim. And it will just restore the data on first day of next month?

That is correct Lebara doesn't offer you credit once used up connection will stop working. Lebara works a bit like pay as go.