That did it!
There were already files in /mnt/sysimage/var/lib/qubes/template-packages
So i just excuted
dnf config-manager --add-repo /tmp/installer.repo
dnf download --downloaddir=/mnt/sysimage/var/lib/qubes/template-packages qubes-template-*
And now it’s fine:) Thank you!
Is this now a valid installation? What issues could arise now?