The default (ubuntu) installer kernel [linux] and initrd [initrd.gz] does not support/know about network booting. Its possible that the debian installer kernel and initrd also doesn’t directly support network booting. Both bits needs to support nfs for netbooting to work. In the screen shots provided appear that the installer scripts are only designed for local cdrom booting.
You may be at a roadblock with this problem.
I did find the debian 9.2 buster netbooting kernel and initrd. I don’t know if you can slide those in place and get untangle to install correctly. I’m sure the netboot kernel will work correctly but the initrd may only install native debian and not the untagle you want. You will want the netboot,tar.gz file. Extract the .gz file in linux or use 7-zip in windows.
http://ftp.debian.org/debian/dists/buster/main/installer-amd64/current/images/netboot/