What membership plan are you on?
Most phones have an option to check what apps are using data and how much. You should check that to see what apps are using most data. For example:
Perhaps it's simply a case of heavy data usage by one or more of your apps exhausting your membership data allowance before the membership month is up?
In which case you'll need to buy a data add-on.
Or switch to the €12.99 membership which gives you unlimited data (subject to 200GB fair usage limit)?
Glad it helped.
So, presumably, if you don't let people use your hotspot so liberally then you won't be running out of data early each month?