I’m glad it works now!
For more recent versions of Ubuntu, there are some manual steps needed. Ubuntu does not provide the boot files on their mirror like it’s been the case in the past - therefore, we must extract the Ubuntu Server ISO image in order to get the boot files. BUT, the new Ubuntu Subiquity Autoinstall mechanism does also need the ISO image iteself.
So, we must provide a) the extracted image files (to set up our installation media in Foreman and retrieve the kernel/initrd file) and also b) the image itself (for Ubuntu’s installation process). If you run Foreman with Katello, you can easily provide all of these files by putting them in your /var/www/html/pub/ directory. For a detailed description on how to set it up, have a look here:
If you run Foreman without Katello, you must provide these files in some other file repo. For the templates using the Autoinstall mechanism, it is just important that the ISO image is placed directly next to the extracted image (which is given as the installation media in Foreman) and has the same naming scheme. You can see what I mean in the GRUB2 Autoinstall template for example: