Peakg,
Excuse me for asking the most obvious...
...You DO have generated an account with the dyndns provider of choice too ???
...Your SME is running gateway&server ?
In case you answer YES on both accounts, please notice
that we rather prefer to have
more information on WHAT you did
(service name, how you setup, how it fails)
before trying to
consult our crystal ball 
Regards
Reinhold