Local files suddenly unplayable on iPhone

[seeBee007]

Local files are a huge headache for me and this has been an ongoing issue for over a year. I have followed every guide on how to sync and play my Local Files on my iPhone, and it has worked once but now it appears that the songs from my Local Files have been "greyed out" from my playlist even though I have never "un-downloaded" my playlist on my phone. Here is what I have done:  

>> Added the location of the songs I own into my Local Files on my windows PC Spotify App  

>> Dragged and dropped the songs from my Local Files into a playlist  

>> Downloaded said playlist on the PC Spotify App  

>> Made sure the songs are playable from the playlist  

>> Downloaded the playlist on my iPhone  

Despite this, the songs in the playlist on my iPhone are still greyed out and are unplayable. I have ensure both my PC and iPhone are connected to the same WiFi, I have given ample time for the songs to "sync" and I have done this process about 3 or 4 times and tried every guide / YouTube video out there but nothing has worked. I'm not sure what my options are now. As a Premium member I expected this process to be a lot smoother. This makes me miss the days of MP3 players when all I had to do was drag and drop songs I liked and that was it. Please advise.