Simple answer, one DNS server should be sufficient.
Is your internet service going directly into your modem/router or is the router connected to your ISP-supplied modem? If it's directly into your router than the info they gave you for setting up PPPoE or if you have a static IP, is all you need. If plugged in to their modem and your router is set to obtain IP automatically, check your status page for the proper DNS servers detected.
Thank you for your answer.
What I want is to access remotely a dvr. I would use the Netgear MBR624GU router with 3G Broadband internet.
As the connection provided by the ISP has a dynamic ip, I asked them if I could use the router DDNS function with dyndns. They answered that they could not provide that function because they only have one DNS. I found I strange, that's why I asked.
That would be the only router I would use.
So in theory I would be able to use router's DDNS function?