If you've subscribed via web, here is how you access on your iOS device.
1.) Download the app at https://apps.apple.com/us/app/marquee-tv-arts-on-demand/id1255573295
2.) Open the app
3.) Select Menu from the Navigation Bar at the bottom of the screen
4.) Select Login
5.) Login using your username and password.
Note: if you registered with email, then use email. Only use Login with Google/Facebook if you originally registered using that method.