Hi,
It sounds like OpenCV in the linux system (not jetson nano) does not enable gstreamer. You may try to run this:
Gstreamer TCPserversink 2-3 seconds latency - #5 by DaneLLL
If this works on the linux system:
$ gst-launch-1.0 udpsrc port=5000 ! 'application/x-rtp,encoding-name=H264,payload=96' ! rtph264depay ! avdec_h264 ! xvimagesink sync=0
You can then try
gst_line2= 'udpsrc port=5000 ! application/x-rtp,encoding-name=H264,payload=96 ! rtph264depay ! avdec_h264 ! videoconvert ! video/x-raw,format=BGR ! appsink '