Hi
As you write. The problem is not the DDNS service. it's the DNS.
of some reason the RRC cannot get in contact with your DNS server. And to be able to update the DDNS server it must first have contact with a DNS server.
Check the DNS IP, if you have a pc connected to the same router you can check what DNS IP it has got, otherwise you can test to use Google DNS 8.8.8.8
73 de mike