There is an answer here on how to connect via mobile broadband using the network manager through the terminal: How to enable/disable mobile broadband from terminal?.
Network manager doesn't come with Ubuntu server as most server installs just have static IPs set on startup by configuration in /etc/networking/interfaces so you'll have to install it.
However, I am questioning why you would do this. I assume you are installing Ubuntu server to actually run a server that can be accessed from the web. Since most if not all mobile broadband providers run carrier-grade NAT, you will not have a publicly routable address under your control and thus no one will be able to access your server.