Try using Roku's own collection of wireless, WiFi mirroring apps found in the Channels menu option, Apps menu option of the Roku box.
When you find one on the Roku box, add it to your channel listings.
Then find a corresponding Roku mirror app, for iOS, in the iOS App Store.
This is how I got my TV app on my iPad to mirror to my TV through my Roku streaming box.
This works, it will still appear on the iPad, but will stream to the Roku box to your TV with a slight delay from iPad to Roku box to TV, but this works.
I have been using this setup to circumvent all of the DRM mirroring limitations that Apple has imposed on the TV app.
This may work for the Fitness+ videos, as well.
Best of Luck to You!