How are you logged into the VOXL to get the window? I was able to get it by running ssh -X root@<VOXL IP> from a Linxu host were I was logged in with a display. I don't have any peripheral to get a display directly from the VOXL.

You can test my method by connecting to the VOXL via ssh -X then running this gst-launch-1.0 videotestsrc ! autovideosink. You should see a window pop up with a test pattern.

Hopefully that fixes the issue, if not, you can try to switch omxh264dec with avdec_h264 but it will then use CPU instead of GPU

Try to run this command on the VOXL-2 and see if it outputs anything when streaming from the host to make sure you actually are getting data into the VOXL:

gst-launch-1.0 udpsrc port=8554 num-buffers=1 ! fakesink dump=true

I also have issues with decodebin receiving an h264 stream as it uses qtivdec to decode the h264 and outputs video/x-raw(memory:GBM) which I have issues working with. You can try this pipeline to use a different decoder:

gst-launch-1.0  udpsrc port=8554 caps = "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H264, payload=(int)96" ! rtph264depay ! h264parse ! omxh264dec ! videoconvert ! autovideosink sync=false