The FQDN is not currently defined for ICE candidates. In RFC 5245, FQDN candidates were under-defined. Specifically, RFC 5245 states that when parsing <connection-address>, an agent can differentiate between an IPv4 address and an IPv6 address by the presence of a colon in its value; the presence of a colon indicates an IPv6 address. This is impossible for FQDN. This means it is impossible to determine the priority for this candidate when the candidate checklist is created, and the transport for the m-line when this candidate is selected. As a result, FQDNs were prohibited when RFC 8839 was written. The intent was that if anyone actually needed FQDN candidates, an extension or an update to RFC 8839 would be written.

I think FQDN candidates are generally a bad idea. Instead of adding an FQDN candidate, an agent responsible for generating SDP should resolve the DNS name and insert the actual address into the SDP..