wi fi – Why can’t my device connect over WiFi?

Here is the steps to connect my Android phone from PC over WiFi:

1.Connect the device and the computer to the same Wi-Fi network
2.Plug the device to the computer with a USB cable to configure the connection
3.On the computer command line type:

adb tcpip 5555

4.On the computer command line type:

adb shell ip addr show wlan0 
#i get the phones ip----192.168.31.86

5.On the computer command line type:

adb connect 192.168.31.86:5555
connected to 192.168.31.86:5555

6.Disconnect the USB cable from the device and check with adb devices

adb devices
List of devices attached
192.168.31.86:5555  offline

Why the status is not online when to disconnect the USB cable from my phone?