Local Files Won't Sync to my iPhone

[17lamrei]

Plan

Premium

Country

 USA

Device

iPhone Xs Max

Operating System

iOS 12.1.3

My Question or Issue

 I have been trying to get a local file that I got onto my computer to sync over to my iPhone but nothing has worked. I have gone through the given steps:

1. Connect this iPhone to the same WiFi as the computer.
2. Launch Spotify on the computer where this song is stored.
3. Mark the playlist containing this song for Offline Sync on your computer or on this iPhone.
4. Watch as your song magically appears on your iPhone.

I have tried this many different ways and nothing has worked.