@nsherin I didn’t use any other files beside the ones came with 18.04.1 LTS. Can you re-extract all files in 18.04.1LTS iso and then reupload them back to /images/os/ubuntu/18.04/mate. You might need to add this line to your /etc/exports file and restart nfs services or restart your fog server.
Internet connection is still an issue with this method. You need to go to /etc/network/interfaces and change from manual to dhcp then restart your interface sudo /etc/init.d/networking restart