Eau Rouge made a good start explaining how to restore Network Diagnostics and Setup, but it's missing a few steps. Here are consolidated instructions:
In High Sierra the Network Diagnostics and Network Setup Assistant apps are no longer in the /System/Library/CoreServices location as they had been previously.
Copy those apps over from a Sierra install, the apps are in the same location as above. To drag them over to your High Sierra install you need to be an administrator as this needs to be Authenticated and System Integrity Protection must be disabled from Recovery (csrutil disable).
Next in High Sierra go to /System/Library/PrivateFrameworks/SetupAssistantFramework.framework/Versions/A/ SetupAssistantFramework hold the Control key down and click on SetupAssistantFramework then from the submenu click Move To Trash. Now move the framework out of the trash and keep it in an appropriately named folder as a backup. Do the same with /System/Library/PrivateFrameworks/SetupAssistantSupport.framework/Versions/A/Se tupAssistantSupport.
Next in Sierra go to /System/Library/PrivateFrameworks/SetupAssistantFramework.framework/Versions/A/ SetupAssistantFramework and copy the framework over to the corresponding location in High Sierra.
Do the same with /System/Library/PrivateFrameworks/SetupAssistantSupport.framework/Versions/A/Se tupAssistantSupport and /System/Library/Frameworks/NetworkDiagnosticsUI.framework.
Change owner of all files to root and permissions to rwxr-xr-x.
Enable SIP in Recovery (csrutil enable).
You should now be able to launch the apps from within the Core Services folder, or make an alias of each and drag them to a more accessible folder.
And don't forget to give feedback to Apple telling them to restore these Core Services to High Sierra!
https://www.apple.com/feedback/macos.html
