@tom.langenfeld I also agree it sounds like an IP address conflict with the FOG server.
Once you determine one way or the other on the IP address conflict I want you to do the following from the FOG server linux console.
(you may need to install the traceroute utility if its not installed by default).
Run a traceroute from the FOG server to a device on this remote vlan where you are getting the tftp timeout. If you have success on a traceroute back to the remote vlan then…
From a windows computer on that remote vlan, see if you can ping the fog server IP address.
If success then install the tftp client program on the windows computer from Program and Features. See if you can tftp get undionly.kpxe from the FOG server.
If success then you must confirm that you have dhcp options 66 and 67 set correctly for this client vlan.