Scheprepleum15 Could be several reasons, is roaming enabled on the mobile? Flight mode deactivated? Do you have correct provider (See list below)? Do you have the approriate package?
Swisscom international tariffs - int. tariff check | Swisscom
Important: If you have a prepaid or a subscription that does not include roaming, you must purchase a data package in advance, otherwise roaming will be blocked. In this case, a cost limit must also be set in the cockpit under Costs before the journey.
In the event of problems such as in this case, you can also try the following: Restart the smartphone. If this does not help, go to the settings and manually select another provider.