iinet Technicolor TG789vac v2 NBN modem can’t connect to DynDNS service

Note: I use specifically (now, this may or may not effect other services.

When setting up the modem’s dynamic dns updater, I kept getting status: bad auth

I triple checked the authentication details and tried both https on and off.

In the end I had to change the password on my dyndns account to one with no special characters.

Also: The modem only supports one dyndns domain/service. This is unfortunate if you want to keep a few updated to stop them expiring.