I had previously installed all of the RSAT tools using the Add Feature portion of Settings, but was missing DNS. I found I also was missing a few others. So looked into further and found the problem is that the installer is balky. So if you select multiple, they seem to get queued up. Be careful to not interrupt the installer! Or, in my case, you do not have a complete winsxs library of all feature cab files. I am still working on getting a complete winsxs folder with all of the required cab files for RSAT.