I can't log in using my Google+ account
You can register an account through Google+ in the web version of Yandex.Music or in the Android app, but you will not be able to log in to the app on an iOS device through Google+.
If you bought a subscription for an account registered through Google+, change it to a Yandex account. Just go to your Yandex.Passport page and tap Create username. In this case all your settings and recommendations will transfer to your new account.
If you can't change accounts, email customer service and we will transfer the paid days to your other username.
It is not possible to transfer tracks that you downloaded. You can, however, copy your playlists to a different account.