Since "n-Track Studio 8" (now named "n-Track Studio 9") has been published as a brand new app, the old version of the app is not available anymore for purchase, but if you already purchased it you can continue using the old version forever and you can still download it.
iOS
Please see the App Store instructions on how to restore purchased apps. Note: due to a change in the iOS graphic system n-Track Studio 7 is not compatible with iOS 12. We are no longer able to release updates to n-Track Studio 7 to fix the compatibility issue.
Android
Just follow one of these two methods:
Method 1
Open the Google Play app
From the left panel menu, select "Account"
Go to "Order history"
Find the n-Track purchase record, and tap on the "View" button
Method 2
Open the Google Play app
From the left panel menu, select "My apps and games"