Hey, I've always had trouble with dynu groups and synology playing ball. Sometimes it'd work, other times not. But I think I've just found a simple solution, which other's might find useful too.

Use this format for the query url:

https://api.dynu.com/nic/update?hostname=__HOSTNAME__&username= YourDynuUsername&group=__USERNAME__&myip=__MYIP__&password=PASSWORD

I think this works because, while synology only has variables for HOSTNAME, USERNAME, PASSWORD & MYIP, we can still pass other things to dynu by hardcoding them into the query url. So in this case, I'm hardcoding my Dynu Username into the url, and use the USERNAME variable as the 'group name' instead.

So once you've set this up, in Synology you only need to enter:

HOSNAME = the dynu hostname you're trying to connect
USERNAME = the dynu group that the hostname belongs to
PASSWORD = the password for that dynu group.

Seems to work so far. Hopefully there's nothing dodgy with it.