Don't want to barge in one someone elses problem and solution, but maybe it helps if you know:
I have the same problem.
I'm located in the Netherlands and run dlinkddns on a dir-655. This has been working for years now, but recently I discovered the IP is not updated. Some investigation showed that the connection with dlinkddns is dropped. If I power down the router and power up again, connection is established, but dropped again after a few hours.
I also own a DNS323, which also has a dddns client. I tried to switch to this client, but the same problem occurs.