Hi Cristian,
Don´t we all love these Chinese cams 
Everything looks very promising. ONVIF set up the stream type to either rtsp over tcp or rtsp over udp. Probably this is the problem. When you have saved the configuration and get the Connection failed go back to the tab Custom URL. There you should have stream type and Address. Change the stream type to rtsp_tcp or rtsp_udp depending on what default is and save. I hope this helps.
Use the latest version 1.5.6 of NCS. Netcam Studio 1.5.6
The URL to the camera is
rtsp://admin:PASSWORD@IPCAMERAIP/onvif1
OR (quality difference)
rtsp://admin:PASSWORD@IPCAMERAIP/onvif2
You can add these manually in Custom URL and test with rtsp_tcp or rtsp_udp and see which one works.
-Henrik