Is it possible to download an offline install DEB package and install Dietpi when I don’t have internet access?
DietPi is not available as APT (deb) package. To run the preparation/install script you need internet access since packages are upgraded, code and new packages downloaded/installed.
You can flash one of our offered images and boot the device offline, but first run setup (which includes an update to the current version + in case some additional package installs, based on CPU (Intel vs AMD) or other chosen settings (dietpi.txt). At some point of the first run setup, if internet access fails, you can cancel it, but the DietPi scripts will then not be fully functional (the underlying Debian/Raspbian system itself of course is).
So the best thing you can do, as far as I can think of now, is to flash the image onto an SBC with internet access first and finish first run setup there. Then you can clone the SDcard/image to flash/boot a ready-to-run instance onto offline SBCs. But of course the offline devices need to be the same then.