Trouble syncing local files from PC to iPhone

[MoosMas]

Hi,

I have some MP3 files on my PC which I play through Spotify, but I want to listen to them on my iPhone as well. When I create a playlist on my PC with those files, the playlist appears on my iPhone (even with the cover) but the songs don't appear. When I play them on my PC I can skip them or scroll through them using my phone, but I can't get them on my phone itself. When I try to download the playlist it still doesn't work, as it doesn't download anything because the playlist is still empty on my phone.

How can I fix this?

Thanks in advance