Due to this post being referenced recently, I'd like to clarify that RADIUS hostnames are only usable when configuring RADIUS Authentication with a Sign-on Splash Page.
For other RADIUS server configurations (such as Client VPN with RADIUS authentication, or RADIUS Authentication with WPA2-Enterprise) only IP addresses are accepted; using a hostname is not currently supported.
If you found this post helpful, please give it kudos. If my answer solved your problem, click "accept as solution" so that others can benefit from it.