The TV Cast for Roku app is fine, as others have indicated and is noted in the TV Cast for Roku app when you first install it, the Roku software update has a broken TV casting ability. By default, my Roku 3500X streaming stick (I have three of them) checks for software updates automatically, so like mine your TV Cast may have been working just fine the one day then suddenly stopped working. Ive tried rebooting everything, switching my WiFi from 2G hz to 5Ghz even changed my WiFis channel to a less-used one. The only way this can be fixed is for Roku to *release a software update for their devices* that fixes their broken ability to cast to the Roku device. Cmon Roku developers! After 2 broken updates you cant (or choose not to) fix this?
Calif48 about TV Cast for Roku App, v1.8