In order to be able to use mobile internet abroad, it is necessary to make adjustments to your mobile device that depend on the operating system.

If you have a phone with an Android operating system, you need to make the following settings:
Select Settings - Connections - Mobile Networks - Access Point Names.
Click on Add in the upper right corner.
Internet is entered in the Name, APN, Username and Password fields.
You do not need to adjust the other fields, that is, they must remain empty.
In the upper right corner, press the three dots and select the Save option, the settings will be saved and you will be returned to the APNs field.

If you are using the iOS operating system, the manual settings are as follows:
Select Settings - Mobile Data - Mobile Data Network - Access Point Names.
Enter internet in the APN, Username and Password fields.
The settings will be saved after you go back to Mobile data on the top left.

Additionally, check in your phone whether the "Data Roaming" option is enabled.

Make an additional check through the "My A1" application if you have a roaming ban or roaming internet ban set on your number.