mkatoma
I have never been able to use Android's native cast (Cast Connect) to cast through my Roku devices. Some of my previous Android phones were able to use Google Cast to cast some content (ie. Youtube, Netflix, etc-- that symbol that allowed user to cast to TV from within the opened app.)
I just use third party casting apps now when needing to cast content from my Android mobile devices to the Roku device. My personal preference is Web Video Caster (WVC), but others like Castify and Tubio work fine also.
With all the above, you install the app on your mobile device or phone (all can be found in the Play Store), and then you install a companion receiver on your Roku device. (all found in the Roku Channel Store).
The apps mentioned above are free (with ads on phone), and all offer paid features also. WVC is a one-time purchase of $3 or $4 for an ad-free full featured lifetime use, but the free version works perfectly fine. (a few additional features with the paid version).
From the Roku Channel Store (companion receivers)...
Web Video Caster
Castify
Tubio