Have you tried ODM ?
If it is truly ONVIF-compliaint, when you login with ODM using the cams's IP/user/pass then the RTSP URL should be revealed on bottom right screen when you go to "Live Video" in middle screen.
So ODM doesn't work/find my cameras. But so far I have the video displayed BI on the first lens.(and the okam app all 3). One thing I did do is use a 9-digit password for the camera. I think you have 2 or 3 passwords in the apps and camera.