Based on my understanding,
Your IP DHCP Range changed from 10.140.252.x to 10.140.190.x?
Was this an on the fly change?
Did you make the changes to your FOG Server configurations? TFTP, WEB_HOST, FTP?
Have you restarted your fog server since DHCP took over? Or at the very least the apache server?
The reason you’d not be able to access the web server (10.140.190.82/fog) sounds like you changed the network but did not restart the apache service.
You’ll run into all sorts of other issues though unless you make the configuration changes. Many of them will be on the Web GUI, but you’ll likely need to make the change to your /tftpboot/pxelinux.cfg/default file as well. All references to the old IP will need to be updated to the new server address.