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