Radio offline

You need magic. If the phone has no internet connection, it will not supply you with radio stations. There is no radio receiver inside the phone. No app can supply a radio connection. To listen to the radio without an internet connection: buy a radio. There are radio receivers you can plug to the iPhone. Go Google.

The only way to listen to radio on any iPhone or iPad is to stream radio data to an app. Without an Internet connection, that cannot be done.

The best you can do with no internet connection is download stored content like podcasts to listen to later when offline.