A UCS server which should under no circumstances have a connection to the Internet needs to install an app from Univention AppCenter.
For Updates you have created a local repository (ie with wget) where the server can connect to.
The possibility depends on the app itself. Some apps are designed as UCS-components and therefore easy to install.
Especially docker apps are much more complicated and for these type of apps you need to create a local appcenter which then allows the installation of apps offline. The creation of such a local appcenter is not the focus of this article here.
The Admin-Diary by example is an app which is designed as a component of UCS.
Therefore the installation should work with univention-install:
univention-install univention-admin-diary-client univention-install univention-admin-diary-backend univention-install univention-management-console-module-admindiary