FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article provides some basic troubleshooting steps where an administrator encounters a failure on update even though the unit has made a request to FortiGuardDDNS with a new serial number.

Generally, the domainname must unregistered on previous unit for security reason.

Failure to update FortiGuard DDNS after hardware replacement.

Make sure that the correct configuration has been set on the unit and run "diag debug application ddnscd -1" command.

Check the logs which describe the request that was sent to the FortiGuard server giving required data such as serial number, domain name and ip address.

If the cause of the failed update cannot be determined, and the issue continues with the unit, then contact technical support and provide the following information:

    DDNS domain
    Old SN
    New SN
    Configuration file
    Result of debug command