After contacting D-Link support: The DCS-6100LH uses RTSP embedded in HTTPS which is sent to the MyDlink app through their servers. For full functionality, the ports 80, 443, 2047, and 2048 should be open.
They say that you cannot connect directly to the camera with RTSP, but you have to use the app. However, the data stream is in fact RTSP, it’s just not using a normal protocol. So it must be possible to extract the RTSP-data-stream but the support were adamant in saying that it is not allowed to use anything else than the MyDlink app to view video streams from DCS-6100LH, and if you want to use RTSP you need to buy a different camera.