My GoDaddy! certificate is not trusted by iOS devices but it is trusted by Android and Windows devices
This means that iOS does not trust the publisher of your certificate, but as you've mentioned, there is a GoDaddy root certification in the iPhone.
What this probably means is that your RADIUS server is not sending its intermediate certificates. The Windows and Android devices probably already have this intermediate certificate trusted, but your iOS devices don't.
You can find someone else having the same issue here.
There isn't much you can do about this, apart from pushing the intermediate certificate into the trusted publishers store on the iOS devices.