For those of you having the "Unable to Connect" error, this is the best explanation I've found - via @downcastapp on Twitter:
"Many audio apps still use the 'com.appledeveloper.playable-content` entitlement, which allows you to provide metadata that CarPlay uses to generate the UI for your audio app. That was deprecated in iOS 15 and, apparently, obsoleted as well. A newer/better method of generating a UI in CarPlay was introduced in iOS 14. It requires a new entitlement: 'com.appledeveloper.carplay-audio'. Despite both entitlements being for audio apps, the new one isn’t automatically assigned to apps that already had access to the older one. Developers of such apps must request access to the new entitlement. While we did this some time ago, Apple has yet to respond. While the app has already been updated to use the new CarPlay framework, it can’t be tested on a device or submitted for review until Apple grants us access to the new entitlement. As soon as that happens, we’ll get the update out as quickly as possible."