I have an iPhone. TuneIn allows me to cast. The TuneIn window has the Cast icon just below the pause/play button (next to the AirPlay icon). The Cast popup window shows the one CCA and the Group I have set up (my second CCA is turned off or it would also show up on the list). I run Google Home to set up the group. But the CCA shows up on my iPhones without Google Home installed.
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I assume you mean Radio Paradise when you mention TuneIn, as that's what your image shows. Again, I have no issues with TuneIn, but RP doesn't doesn't present an option to cast.
So strange that I can't get mine to work. Maybe I'll try setting something up within Google Home.