iphone doesn’t find Spotify Connect

[Kolmarden]

I have an Argon Stream 2 with Spotify Connect. It works fine with my Android smartphone (OnePlus 3). My problem is that none of my other 3 family members with various iPhones see the Argon Stream option in their Spotify Apps. I’ve come to the conclusion that it can’t be anything wrong with neither Spotify Connect nor the Argon Stream 2. How can I get it to work on their iPhones?

Thanking you for your help!

Carl