I have tried using my account password, ip update password, and Md5 and SHA256 hashes for both passwords, and authentication keeps failing.

I am using the script for synology devices, the DDNS client.

What is wrong with you service?

I also have this problem. Waiting on a response now.

You need to make sure that the domain name you are updating is a primary domain name instead of a subdomain created under a primary domain. In other words, whatever domain name you are updating needs to show up in your domain list at https://www.dynu.com/en-US/ControlPanel/DDNS.

The reason for that is the Synology uses the below update URL:

http://api.dynu.com/nic/update?hostname=__HOSTNAME__&username=__USERNAME__&myip=__MYIP__&password=__PASSWORD__

And the subdomain's (sub.domain.com) update URL is different:

http://api.dynu.com/nic/update?hostname=domain.com&alias=sub.......

The issue is resolved for me. I created a location and had to update the client with the location information.