Wlan with Radius: Radiusserver only with IP, not with name


in the Interface Configuration / Wireless Security I have to enter the ip for the Radius server.
But I have to use the FQDN, otherwise we have the problem that the certificate from our Radius server does not match.
No name, no certificate. No certificate with an IP.
I can enter the name, but WIFI stops then. Until I change back.

Does anyone have an idea how to solve the problem?
Thank you in advance...