@Zelmo
Using Google Cast actually tells the destination device, your Roku in this instance, to launch the app in question. Google Cast does NOT stream through the mobile device. The mobile device is telling the destination device to launch the app. Most people don't realize this is how Google Cast works for what you're trying to do. It won't work for any device that doesn't have the corresponding app.
The question becomes why doesn't your Roku device support the Watch TBS app.
When I go to Add Channels in my main menu (or Streaming Channels in the left column), then scroll down to Search Channels, I can type TBS and Watch TBS shows in the search results.
You say you don't see that. So, that leads to why that's the case. And I have a couple of questions.
Are you in the USA? Physically in the United States of America?
Are you using a VPN?
The Watch TBS app won't work or even show up in an area where Turner Entertainment Networks doesn't approve Roku offering it.
DBDukes
Roku Community Streaming Expert
Note: I am not a Roku employee.
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