If you (or the script) don’t provide a protocol, then the app protocol is used, and Capacitor apps use capacitor as protocol.
So urls like //www.radiojar.com become capacitor://www.radiojar.com, and they are not reachable since they are online and capacitor:// will search in your app assets.
You should use the https url as you already did and contact radiojar so they do the same in their internals.