I have an IP2M-841, and I have DDNS set up. I have been able to make web (Chrome & IE on Windows), tinyCam and Amcrest View Pro (the latter two on Android) work externally, both video stream and control for all. For web & tinyCam I used HTTPS. For Amcrest View Pro I've used P2P and IP configs (of which the latter is definitely more reliable.) I only needed to set up simple port forwarding on my router. Following are the details:
1) web: Although I figured out how to make this work, I would not recommend it. Besides forwarding HTTPS, you also have to forward HTTP. Which means that someone could log in insecurely. Worse, even if you do connect initially via HTTPS, to get the video stream (and not that useless limited resource error message), you have to use 80 for the external port number for HTTP. There seems to be no provision for specifying a different HTTP port number. So, if you have more than one camera, well... (I guess you can use the proxy methods previously described.)
2) Amcrest View Pro: P2P "just works" ... well, most of the time. This method doesn't seem very reliable. To use an IP configuration instead, you simply need to forward the "TCP" port, and you can use any open external port number. So, you could easily support more than one camera with different external port numbers.
3) tinyCam: This requires only the HTTPS and RTSP ports to be forwarded, and again, any external port numbers are ok (well, other than reserved ones.) In tinyCam, on the advanced camera settings page, enter the external HTTPS port number in the "web" port number box, and the external RTSP port number in the corresponding box, and check the "Use HTTPs" box.