In addition, locating a device is associated with effort

Theoretically, it is possible to gather information from mobile operators about which cell a certain device is located in (and thus, approximately, where it is), but in reality this is difficult. Surely you know that you are not allowed to query the location of any cell phone “just like that”. We are not aware of any provider in Germany who would offer such a query as a service for companies, even with explicit consent of the recipients.
But even in cases where the query is permitted, it would quickly become very expensive if hundreds of recipients were involved.
Be prepared, however, that this version might not give you the desired goals. On the one hand, the hurdle for recipients to download your app just for this purpose is big, on the other hand, this kind of location tracking often consumes a lot of battery, which is why your app might be deleted again quite quickly.
The bottom line is: this type of geofencing marketing is costly to implement, if it’s possible at all.
Another way to implement geofence-based SMS marketing:
Your customers download an app to track their location and then allow you to access that location there. Be careful, you should additionally obtain their consent to receive SMS. You’ll have to decide for yourself how useful it is to send SMS if your recipients have already had to download an app.