How can I listen to DAB+ radio on my iPhone?
I offer this answer as a possible software solution to meet your needs, although any software solution will require data usage.

I've just checked and TuneIn Radio is available on the Norwegian iOS App Store. For full details refer to https://itunes.apple.com/no/app/tunein-radio/id418987775?mt=8

I've personally been using TuneIn Radio for many years now and have generally been very happy with it. It provides access to over 100,000 radio stations and even offers users the ability to submit station requests in the event a station you want to listen to isn't available.

There is also a Pro version available (obviously at a cost).

NOTE: I am not affiliated in any way with TuneIn Radio.