You are describing some sort of reverse casting/mirroring scenario that isnt available.
If you want to use any of the apps that you have installed on your Roku on your iPad instead, you would simply use the native apps on your iPad (assuming they are available for iOS).
E.g. if you want to use NetFlix on your iPad, just download and install NF from the Apple store and use it.
Native app playback is almost always preferable to casting/mirroring, so this isnt something you want (unless the app was available on Roku, but not for iOS).