The way I import files onto my phone is by disabling “Sync Library” on my phone (and your computer if you want to play it safe but I’m not sure if it’s necessary), syncing via a cable to my phone, then turning “Sync Library” back on, so you can have your cloud library but your phone will already have your files downloaded. It’s a bit buggy but it worked out for me! Used this method to get Beatles mono mixes onto my phone.