It sounds like you installed the DHCP server on your fog server, and it’s not communicating properly because your router doesn’t know how to pass the information, to the tftp server. What kind of router are you using? Do you have the ability to setup something like:
DHCP 66/67 information?
Could you try with a switch or hub connected directly to the fog server (bypassing the router) just to see if it can actually communicate with the FOG Server?