Thanks for your response. I've configured a static route as per the second bullet point above. Is that what you mean with routing and FB knowing about the second network? I couldn't find an option to enable remote HTTPS.
If I look at this and this topic (you were able to help with both) - it seems I'm having the exact same issue. Only that in the new 4.6 software the presented solution can't be applied anymore (in the same way).