Firstly, are you sure speed is from the sensor not GPS? make sure GPS is off so that you're certain it's sensor data. If speed doesn't show it may be a battery issue in the sensor. My next check would be the crank magnet position. If the 820 has found it as a speed/cadence combo sensor then the sensor would appear to be working. If the magnet looks right, try passing a magnet closer to the frame where the sensor is or even take the sensor out of the frame for a test.
I spent hours trying to get to get my new Explore 820 to 'find' my existing cadence/speed sensor which works fine with my 500. I was trying to do it through Cadence to no avail. I just tried All Sensors with the cadence being the only one active and it found it straight away!!! I had also turned GPS off some try's earlier and it was still off when I got success - don't know if this also helped.
Because the sensor was a combined sensor (speed and cadence) you needed to search for speed/cadence and not cadence. Searching for cadence will only find dedicated cadence sensors Once you changed your search to "search all" it found the combined speed/cadence sensor.