I finally have gotten it to work! I have my local files from Youtube on my laptop's Spotify. I set up a mobile hotspot on my phone and connected my laptop to the hotspot. Then I moved each song individually to a playlist, downloaded the song and then downloaded it on my Spotify for mobile. Don't delete the songs from their individual playlists, just move it to a playlist with the rest of the local files. Otherwise the local file songs will not play.
Hi Calibrations39! If you still cannot solve this problem, try downloading audio files with a Youtube to mp3 converter. This will work 100% and it's easy to do. This method is available for both Android and iOS phones. You can copy-paste MP3 files from your PC to iTunes, did you know that? It's kind of boring if you want to download a bunch of audio files, but it's easy to get used to it.