I did all the instructions correctly but screen Mirroring just doesn't work. I made sure screen Mirroring option is on "Always allow." But when I get to my Android phone, Roku 3 doesn't show up when I click cast. I'm using a Moto g6, so I'm not sure if that has to do with it.
Not all Android phones will cast to non-Chromecast devices, especially if, like Moto phones, have stock or near-stock Android. I have a G5+, and it also will not see any cast device (Roku or even myBD player.)