See this link : http://www.talktalkmembers.com/t5/Unlimited-Broadband/DDNS-settings-on-HG533/td-p/844595
It says: ...Unfortunately, there is a bug in the GNUDip.http implementation on all Huewai TT routers - the salt, sign and pass parameters are not correctly passed to the server - they are sent, but missing the last character in each case. So, with an incomplete password hash sent by the router, the remote server will be unable to verify it Smiley Sad. That's probably why you're having trouble with it.
Then: ... on 11-04-2015 02:11 PM
I notice the bugs in the GNUDip.http implementation are fixed in the HG533 v1.18t firmware - that is the salt, sign and pass parameters are now passed in full, whereas these missed the last character with v1.15t. Also, v1.18t now correctly only accesses the update script when the IP address has really been changed, and not repeatedly forever every minute or so as was the case with v1.15t.
If you can change the firmware good. Try and let us know.