@JoeMartin 

Thanks for reaching out. I understand that you are experiencing issues with the DDNS feature on your Deco Mesh system, specifically with connecting to your DynDNS provider. Although your credentials are correct and you can log in to the DynDNS website directly, the Deco app displays a 'connection failed' error and cannot save changes.

To clarify, Dynamic DNS (DDNS) allows you to access your home network remotely using a fixed domain name, even if your public IP address changes. Deco Mesh supports DDNS providers including TP-Link, DynDNS, and NO-IP.

Here are the recommended steps to configure DDNS on your Deco Mesh:
1. Open the Deco app and navigate to MORE > Advanced > DDNS, then enable the DDNS feature.
2. Select your preferred DDNS provider (such as DynDNS) and enter the required information (username, password, and domain).
3. If you encounter connection issues, double-check your credentials and ensure your main Deco's firmware is up to date.
4. If you do not see DynDNS or NO-IP as options, your Deco model may not support them yet. Please update your Deco's firmware to the latest version.

For more detailed instructions, please visit: https://www.tp-link.com/en/support/faq/3481/

Notes: If you continue to experience connection errors after confirming your credentials and updating your firmware, there may be compatibility or service issues. You may want to try removing and re-adding the DDNS configuration, or contact TP-Link support for further assistance. Be sure to mention your TP-Link ID, Deco model, and firmware version when reaching out.

Let us know if you need additional help!