Starting with Windows Server 1809 RSAT is no longer a downloadable option. We see this on the Microsoft download page here.
So how do we install the tools? The easiest way I found to do this was to click on the start menu and search for "Apps & features". On this screen click on Manage optional features. This screen will list all the items that are already installed. To add a new feature simply click on "Add a feature. I installed the following two:
Once the feature has finished installing you can find the tools on the start menu, under Windows Administrative Tools.


