Making Ubuntu 18.04 strictly look to preseed file for boot via PXE.
After installing dnsmasq, use the included config file to enable tftp, dhcp, etc. The location for tftp is also identified in the config.
Place the grub config as /srv/tftp/grub/grub.cfg
Debian Preseed - The most useful resource when identifying kernel boot paramters.
Place the included preseed file /srv/tftp/.