I have exactly the same issue. Q75R 55". The TV only appears in the 'Cast' menu options on my phone (Galaxy S9) when I use the YouTube app. I notice that it appears with "[TV] .." prefixed to the TV's cast name. All other apps that support 'cast' don't list the TV as a cast option. I'm guessing that Samsung haven't implemented a full Chromecast functionality in these TVs (despite what the advertising leads you to believe). It appears to be a locked-down cast solution that only works in conjunction with 'SmartThings'. For me this renders the built-in 'cast' useless. I can't cast from Photos. I can't cast from VLC. I can't cast from any of my phone's TV catch-up apps (e.g. ABC iView here in Australia). I was hoping with this new TV I could ditch the old Chromecast dongle, but I have it plugged back in now, and it's working as it should.
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