So far, everything looks good. I wanted to make sure you were using the IP address of the fog server and the right boot file name.
I understand about Orange (silly name change in my opinion), we used them about 10 years ago for WAN management.
Using the fog dhcp server is not mandatory and should be a second choice unless you don’t have a functioning dhcp environment.
The error message is for sure saying that it can’t reach either the fog server or the download file.