Downloading files from the web on iOS has always been a prickly issue, but Michael Rockwell, writer at Initial Charge, has come up with a rather simple solution:
One aspect of the iOS-first lifestyle that has been a bit of an issue, for many users, is dealing with files. Apple has done a lot to try and fix that with the Files app in iOS 11, but it isn’t fool-proof. One common pain-point for me has been trying to download MP3 files that exist behind a paywall. Luckily, a few must-have iOS apps are available to smooth out the rough edges.
I subscribe to Wrestling Observer to gain access to their premium podcast content — because I’m a nerd who likes pro wrestling. I prefer to listen to these shows in Overcast so I can use the app’s Smart Speed feature and play it back at about 1.2x, but Overcast doesn’t have support for password protected RSS feeds. I’m holding out hope that this will be added in the future, but until then, I’ve been using iCab Mobile to download the audio files, which I can then upload to Overcast’s servers with my Premium subscription. And of course, Workflow is there to facility the process.
Michael seems to always have nifty solutions to things on iOS, so it's worth reading the full article, and maybe even subscribing to the blog in your RSS reader of choice.