What service to use for my (new) domain ?

You need Custom DNS (and a domain registration) for that - the Pro service is for use with the free hostnames.

Contact DynDNS and explain that you've purchased Pro in error. Other posts on this forum suggests they're usually helpful in these situations.

Ok. Thank you for that. I have done that and that is in process.

Other question:

DynDNS only has around 10 TLDs that can be registered (.com, .net, etc etc). On the Custom DNS page they say they support almost every domain extension. When i would register a domain somewhere else (i wanted to do it at DynDNS but they dont have the domain extension i want) is everything just as fast as if i would register it at DynDNS directly? Because i want my own domain, but i also need a fast TTL when i use the updater tools for my dynamic IP`s.

You must differentiate between Domain Registration and DNS Hosting Service. You must have both for your domain functioning, but do not need to have both at DynDNS. You can manage more TLDs with their DNS hosting service than you would be able to register with their domain registration. So yes, you can register your domain somewhere else, and you can use their DNS hosting service DynDNS Custom, especially as you have a dynamic IP address and want to have total control over your DNS (e.g. TTL). If everything is "just as fast as if i would register it at DynDNS directly" depends on so many factors that this cannot be answered. But because DNS in general (as the phonebook of the internet, not involved in speeds and connections) is rather fast anyway, I would not really expect troubles.

Ok thank you for this clarification. What i meant by: if it was as fast as DynDNS itself was more that i meant that it does`nt take as long as a normal DNS entry to be resolved by all thye DNS server. I hope i explained it correctly.

Thanks for the info.