Is our router connected to the internet? it is possible to use a router to create a network without it being connected to the internet. You could also try renewing and flushing dns

Select Command Prompt (Run as Administrator).

Type ipconfig /release in the command prompt. then press enter

Wait a few seconds for a reply that the IP address has been released.

Type ipconfig /renew in the command prompt. then press enter

Wait a few seconds for a reply that the IP address has been re-established.

Type ipconfig /flushdns in the command prompt. then press enter

Close the command prompt and attempt to make a connection.

Hope this helps