I just configured my new chromecast audio and have been unsuccessfully trying to connect it with Spotify Premium on my iPhone 6S. I'm on the same network, the chromecast audio shows up in the Google Cast iOS app, and I can stream audio to it from Chrome on my laptop, but the device has never shown up in the "Connect" tab in Spotify iOS. I have the latest version of Spotify installed, and am a premium user.
Spotify iOS version: 220.127.116.115
Chromecast audio firmware version: 1.18.55065