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.