Problem Playing Music on Denon HiFi After Pairing

I'm asking this on behalf of my sister.

She's tried to pair a Denon D-M41DAB HiFi with an iPhone 15 as per manual. However the display on the HiFi says "Pairing" and a notification on the iPhone of a pairing request doesn't appear. Eventually the pairing attempt times out. However it's possible to find the HiFi as a Bluetooth device on the iPhone and it can be connected to from the phone. The HiFi then says that it's connected to the iPhone, as does the iPhone. However when music is played from iTunes, it doesn't play on the HiFi, just locally. I was working on the idea that maybe the output device had to be chosen on the phone as shown in this YouTube video, but she couldn't find a symbol in the play panel in Control Centre for picking an output device.