I had the exact same issue 2 days ago, trying to get my old Core working again on my new WiFi access points. Give up on the phone app, the best way is to use the Command Line Interface (CLI) and go in via the DFU method. It seems difficult, but once you sit back follow it correctly, it should work fine.
Take a look at my old post, which references several other great posts.
The only thing, the dropbox links no longer work, and ended up using the “particle doctor” suggested from this post.
Good luck!