What Roku refers to as "screen mirroring" in its menu (Settings/System/Screen mirroring) and documentation is Miracast-based. Roku also supports AirPlay2-based mirroring as well which is in the menu at (Settings/Apple AirPlay and HomeKit)
Apple devices support AirPlay2, whereas Windows/Android devices support Miracast.
If your Roku 2 is a 4205 or 4210, you should have AirPlay2 support w/OS10 build 4195 or later, which is what you would use with an iPad.
https://support.roku.com/article/360057488733
The Roku devices listed below must be running Roku OS 10.0 or higher to support Apple HomeKit and streaming with AirPlay 2.
Device Model
Roku TV | Dxxxx, 8xxxx |
Roku Streaming Stick | 3600, 3800, 3801 |
Roku Express | 3900, 3930, 3801 |
Roku Express+ | 3910, 3931 |
Roku HD | 3932 |
Roku 2 | 4205, 4210 |
Roku 3 | 4200, 4201, 4230 |
If your Roku 2 isnt a 4205/4210 it will not have AirPlay2 support, and you will not be able mirror to it from your iPad.
You should check your model # and firmware version to answer this question: Settings/System/About