How to upgrade Debian 10 to Debian 11 without internet?

In the future I have to upgrade several Debian 10 systems to Debian 11. The problem is: The systems have no access to the internet. What options do I have to upgrade the system? After some research, I found apt-offline which seemed to be suitable for this task. I tried apt-offline on a fully updated example Debian 10 in the following manner: offline system: 1. change the /etc/apt/sources.list from buster (Debian 10) to bullseye (Debian 11) 2. create apt-offline.sig with :

apt-offline set --upgrade-type dist-upgrade apt-offline.sig

on the online system: 3. create bundle.zip with

apt-offline get --bundle bundle.zip apt-offline.sig

on the offline system: 4. install bundle with :

apt-offline install bundle.zip

This does not work. apt tries to fetch packages from the internet when apt-get dist-upgrade is performed and the bundle.zip is also only 27Mb big. It doesn't look like apt-offline is suitable for doing Debian release upgrades. Is there any other method to perform an offline release upgrade from debian 10 to debian 11?