This documentation is for all macOS versions.
Start System settings
Select Network
Under Network add a new connection with
As connection select VPN and as type L2TP over IPSec.
As Service name use StudNet for example
As Server Address use vpn.stw-bonn.de .
Your Account Name is the user name you received by e-mail after registration on https://startup.stw-bonn.de
Click on Authentication Settings and enter your Password, which you set on account registration on https://startup.stw-bonn.de.
After this enter the Shared Secret which is: internet
Confirm with OK
Click on Advanced Options and activate Send all traffic over VPN connection and Use verbose logging.
Confirm with OK
Then click on Connect.