Firstly, you must find the exact url for your video stream, and that's best done with a web browser. For example I use IP Webcam app on android (com.pass.webcam) and it's stream will be on:
http://phone-ip-address:port/video
If I visit that url with a web browser, I can see the live stream. Make sure, that what you see is only the video stream, not a html page with the stream. If there is a html page, you can right-click and select Open image in new tab (in Chrome) to get to the stream.
However it looks like OpenCV can only read the video stream if the filename/url has the right suffix. Adding ?type=some.mjpeg worked for me. So the url would be:
http://phone-ip-address:port/video?type=some.mjpeg
Try visiting such an url in the web browser before you go for opencv.