As of now, the service administration is done via WMI. Reporting Services registers a WMI class on the server, and anything that can access it—including the ReportingServicesTools module, of course—can manage the service through it.

This means that, in order for the module to be able to perform these steps, you need to be able to access the service remotely via WMI/DCOM, unless you are running it on the computer hosting SSRS itself.

To connect to a computer, you can use the Connect-RsReportServer command, removing the requirement to specify the target computer on subsequent calls:

Connect-RsReportServer -ComputerName "sql2017" -ReportServerInstance "MSSQLSERVER"