There I found it, on page 142. Thank You. I think, that belongs into the wiki, since this is the source of information I get routed to when clicking on "user manual" on the product page! It's sad, that a third party has better documentation available... Teltonika, please fix that!
Also, this manual I wasn't aware of, shows, that any push-pull outputs of external devices should not be connected to DIN1. They can go into DIN2, but that has the wrong voltage level. My 3.3V there are still considered "low".
So, ok, now I know what to do: make my push-pull-output into an open collector by adding a transistor.