TL;DR: As far as I can tell with my rather limited French, it doesn’t seem to be possible to install additional apps.
In general, managed Nextcloud offerings from hosting providers are often limited in one way or another, especially when it comes to installing apps. In return, you get an instance that’s fully managed by the provider, so everything is maintained for you and you don’t have to worry about updates or administration.
If you need additional features, apps, more users, etc, the provider is the right point of contact. They may offer higher-tier plans or dedicated Nextcloud instances where you have full administrative rights. Those options typically come at a higher price, though.