I had never heard of the $6.49 premium plan for Lightroom CC on iOS, so I looked it up. The reason I had not heard of it is that the extras that come with that plan are automatically enabled when one has any of the subscriptions for desktop applications that I outlined above. Assuming one has the desktop apps, then in Lightroom CC for iOS one signs in with one's Adobe ID and the features are just there; no further steps are needed to activate them.
In other words, if you subscribe to, say, the Adobe CC Photography Plan at $9.99 you will get Photoshop, Lightroom CC, and Lightroom Classic CC for your laptop and desktop machines*. Then you may download the iOS application and, after signing in to your Adobe account on your iPad, the mobile version of Lightroom CC will be fully enabled at no additional cost.
*You may have Adobe CC activated on two computers at a time, but you may only run it from one computer at a time.