DOWNLOADED SONGS ALWAYS BEING REMOVED

[CITYDOVEY]

Hello I'm using my Huawei P9 plus, and I'm a premium user. I have downloaded all the saved songs but every one or two days it will be "undownloaded" and I have to keep downloading every songs in my library all the time and to use my cellar network to listen while I was on street!! Since I have thousands of songs this process is really using a lot of time and network data (which I already set up to use Wi-Fi network only). How do I solve this?? This is reallllly annoying.
Thanks for help!