I had the same problem.
The red light normally means that the whisperer needs to be charged.
A couple of hours is not enough. It seems that the solar modul charge with very low voltage. Try it with a couple of days in the sun.
In the moment one of my whisperer is also empty. Unfortunately is is cloudy and raining....
"The sensor has been kept in the box for long time and is currently completely out of power.
It will need more time to charge. Please leave the sensor under direct sunlight for about 3-5 days. Then press the power button again.
If you have incandescent light bulb at home, you can also leave the sensor charge under incandescent light."