iMac, Spotify, Google Home and my (paid-for) mp3s. How do I get the music to play?

[DA5809]

Premium

US

iMac 2018/iphone8

iOS 10.14.5

I have been able to get some local mp3 files that I purchased onto my phone and my iMac.  I added them to a Spotify playlist and the local mp3s play on the two OS devices.  However, when I cast this playlist to my Google Home group, it plays the Spotify music but skips the local files.  I can't figure how to get them to play on the Google speaker .