I am quite new with OpenWRT. Maybe I am missing something but every packages I install are correctly installed (No error) but no UI for the package appears in 'Service' Tab. Is there anything I need to do to make it appear? Checking the processes displays it.
That is of no help. You need to tell exactly what packages you installed. For example every package that has a LuCI integration needs two packages to be able to work through Web UI. As in your case you need luci-app-ddns then it should also install the ddns* package with it.