@LadyBugAnita-
The Roku app on a phone is primarily an alternative remote control. It doesn't turn your phone into a full fledged Roku device.
It does however have an additional feature that enables you to view content from THE ROKU CHANNEL on your phone. When you click on a show to watch you will be given an option "Watch now on mobile".
I don't have any premium channels through Roku to check, but reportedly you may only have access to actual ROKU CHANNEL content, not premium subscriptions you normally view through The Roku Channel on a Roku TV or device.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Ultra 4850 | Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X