An email about this went out to all DynDNS Free account holders on or about May 15.
DynDNS will continue to be free. All that is required is you log into your account every 30 days. That's an actual log in, and not the "check/update my ip" thing via the router, DynDNS app or third party app.
This is the log in page: https://account.dyn.com
If it's that important, the Pro service is $10 for a year (see the email they sent you for the discount link). Or if one is incapable of an every-30-days discipline, it's otherwise $25 a year. Using your discount link, that $25 will get you two years, five for $75.
Although it would be nice if the router could be configured for other services!