No-IP added DDNS keys to offer improved security by having unique credentials that are disassociated from No-IP account login credentials. Using DDNS key, we don't have to share No-IP login credentials. It works perfectly fine for a single No-IP record or a single WAN interface. But It don't work with multiple WAN interfaces because all WAN interface needs to use the same domain all.ddnskey[dot]com which is not allowed currently. 

Please read more about DDNS keys here - https://www.noip[dot]com/support/knowledgebase/how-to-setup-and-use-a-ddns-key