Dear george1421 ,
Finally i solved my problem. For Future guys ,
you need to add this line also to your /etc/dhcp/dhcpd.conf file .
next-server and some ip address , like : next-server 192.168.x.x.
my mistake was that i was writing it with “option” word . But next-server is string and it must be without option word.
Thank you for your support ,
This can be marked as solved .
Good Luck to all.

Dear george1421 , thank you for your promp response. DHCP is in another linux server , Fog is in seperate . i will check this link . Interesting part is that it does work if i push DHCP from Windows Server , not from Linux .
beside, my fog works only on bios, Can’t boot from UEFI.
Your advice much appreciated.

Greetings everyone, your help , advice much appreciated. I am using fog 5-6 months. It was just fine with Ubuntu and Windows Server 2012 DHCP .
Now i have installed another Fog server in another place , again used Ubuntu 18.04 Bionic Beaver and Linux DHCP server . My Fog Server gives error TFTP Time out .
Paralel i have installed another Windows Server 2012 and Configured DHCP there ( option 66, 67 also ) . It is fun that , this time it worked . I have successfully cloned and captured image with clone.
My Question is : what am missing in ISC-DHCP server in linux? in /etc/dhcp/dhcpd.conf file i added both 66 and 67 options with “tftp-server-name” and " undionly.kpxe " .
Guys , please it is really important and urgent, your kind help much much appreciated.