I had the same problem. I am using student Premium. Local files on PC added to the playlist, but I can't download them on my phone. I tried to disable firewall, set network to private, reinstall spotify mobile app, didn't work.
Then I realize the problem might be with my wifi. I don't have access to the reuter settings, it is a Xfinity wifi.
So as above suggested, I turn on the mobile hotspot on my phone, and connect my PC to it. I have to also turn on "download using cellular" in spotify app setting. Then wait for a few seconds and I can finally download the local files on PC to my phone.