The knowledge base has only partial answers for DDNS issues, not a full Synology DDNS fix guide.
What it does say:
1. Check whether your Synology device is properly connected to the Internet.
2. Temporarily set the DNS servers to:
- Preferred DNS server: 8.8.8.8
- Alternative DNS server: 8.8.4.4
3. Temporarily disable IPv6:
- DSM: Control Panel > Network > Network Interface
- Select your network interface and click Edit
- Under IPv6, set IPv6 setup to Off
4. If your device is joined to a domain, make sure the domain DNS server can resolve the websites required by DDNS and External IP Analyzer services.
5. If DDNS still cannot connect, the knowledge base says the NAS may not be able to connect to the DDNS server, with the detailed message:
- “Connection failed. Cannot resolve the IP address of the server.”
6. If you cannot update DSM in general, similar troubleshooting steps include:
- Check whether the upgrading process was interrupted
- Check that you are using a newer DSM version
- Download the update again because the file might be corrupted
The knowledge base does not provide a more specific recovery procedure for Synology DDNS updating the IP beyond these checks.
Sources used: [9], [10], [11]