DHT11 is not reading Temperature and Humidity when Arduino is powered on using DC power Supply (12V)
You haven't really provided enough information to answer. What are you reading the temperatures with. You code just assigns a variable to the readings but does not output that to anything.

I would suggest reading along with this tutorial, for example, and trying out the code they provide: http://www.circuitbasics.com/how-to-set-up-the-dht11-humidity-sensor-on-an-arduino/