DynDNS is a service that allows you to have a static domain name even if you don't have a static IP. You have to :
Several modems (NeufBox, …) only support dyndns.org, which makes it impossible to use a real domain name bought to a real registrar.
Fortunately there is a trick : you have to add a DNS entry in your modem for making “dyndns.org” point to the IP address of your real provider.
Source : dyndns.fr
Needs testing with OVH.
Problem: Neufbox 4 limits the user name at 15 characters, and the domain name at 20 characters…