data recovery - ADB: Phone is not recognised but Ubuntu 18.04 recognises it

My LG G3 phone (phone is rooted; OS is Android Fulmics) is recognised by my laptop when I plug it in. The laptop is running Ubuntu 18.04 and I can easily copy data from and to my phone.

I have recently installed the Android Debug Bridge (ADB) (sudo apt-get install android-tools-adb). Then, I wanted to reach my device and typed adb devices.

When functioning properly, it should return something like this:

* daemon not running. starting it now on port 5037 *
* daemon started successfully *
List of devices attached 
5200d6fd904b2200    device

But when I tried it, it returned this:

List of devices attached

* daemon not running. starting it now on port 5037 *
* daemon started successfully *
 

So, List of devices attached appeared first and no device was listed.

When I type adb devices again, I get this:

List of devices attached
 

I have also tried to use sudo adb devices instead of adb devices but the results were the same.

When the ADB would work, I would try to use it for data recovery purposes as described here. Maybe the pull command can do what I want.

Answer

This has solved my problem. I could not turn on USB debugging because the phone was connected to my computer. You have to unplug the phone first, start the USB debugging and plug it back in. Then, adb devices worked.