Try the New Ubuntu Server Installer
The new Ubuntu Server installer is now supported on all Ubuntu Server architectures and features unattended autoinstall; offline installation; network-gapped install; PXE and HTTP boot; and a full range of filesystem configurations including RAID, LVM, and LUKS.
Netboot guides are available for using the new installer on various architectures:
Get help migrating to the new Ubuntu Server installer on Ubuntu Discourse!
Legacy Ubuntu Netboot Images
Netboot images of Ubuntu are available for the following releases: