During the Travel Data App installation, it should have requested you to turn on Notifications for the App. If you did so, you will receive notifications as your data is getting close to 0 MB balance or if the duration of your plan, in days, is about to expire. You can purchase a top up plan if you like at that time.
To check if your notifications are set:
- Go to Settings
- Tap Notifications
- Scroll down to Travel Data and select it
- Allow Notifications should be turned on. If it is not, please turn it on.