I believe you have to modify the client OS to support being booted over iSCSI, though I have limited experience with it.
I could be wrong, but I think by default iSCSI isn’t included in the initramfs of Ubuntu.
Perhaps this https://apfelboymchen.net/gnu/notes/booting ubuntu iscsi.html could be helpful. (at least the client info)
All that said, NFS boot is likely to be a lot more straightforward to implement and I use that method quite often.