I was also able to solve the other “problem”. I can render the data from the other DB just fine.
It helped me a lot to watch a complete tutorial about Django on Udemy. There someone explains all the relevant things step by step and in great detail. Before that, I just looked at YoutTube and took over something here and there. This does not lead to success in the long term.
Now I know how to test my “commands” in advance on the shell and thus I can debug a little.