is there a way tp backup the system including all installed packages and config so i can "install/upload" this one file on uboot?
I might not have internet at the reinstalling spot so no way to download the before installed packages like "installed-list" i have read before.
Thanks for your help
This has been asked, and answered, many times, the forum search should provide you with further details.
But, the short -and only reasonable- answer is "no".
A more exhaustive answer would, among other things, be heaviliy hardware specific - and still end with "don't, just don't".
share internet from your cell phone (or some other wifi), have the router join the hot spot, download the packages you need, enjoy the end result.
or use image builder to create a customized image, including your extra packages.
depends on you router and the format of you partitions
but you can backup your firmware "kernal & rootfs etc"
but the way you restore this depends on you router
but it will return to the state of the backup