IOS ( iPhone / iPad) - Music App - Library on external device

I want to play my music on the IOS Music App on iPhone/iPad and store the music library and files on an external drive. My current library is about 400 GB. I can run this on my iPhone/iPad but I will shortly be running out of memory. So I would like - as I'm proceeding in MacOS - to store the music library and data on an external device.

Connecting my iPhone to an external device, I can access each song individually but I cannot access the library. As a consequence I not able to use the shuffle option nor play a cd (if the songs are stored individually).

Anybody hints? Thnx.