Network Diagnostics Replacement Network Diagnostics was removed from Mojave. The replacement does not tell you if the failure to connect issue is with the Internet, or your own WiFi. The program could often fix connections where the replacement can only give a report. 

Is there a (3rd party) replacement for Network Diagnostics?
The first party tools are listed here:

https://support.apple.com/guide/mac-help/use-network-utility-on-mac-ntutl001/10.15/mac/10.15

Most of the data is now in the network preference pane.