Nine iOS stopped its development, and we no longer sell its license from the App Store. So you can't download the Nine iOS app from Apple App Store anymore. But even though Nine iOS has been missed out on in the App Store, you can install it by searching in the App Store on your iPhone if you purchased the license before from the Apple App Store.
On your iPhone, Go to App Store > Account > Apps > You can find it in the section "Not on this iphone".