My devise always has offline status

Hello,
I recently started using tracing. I created an account in demo.traccer.org to test the operation and I downloaded Traccer Client on my phone.
I put as Server Url (http://demo.traccer.org:5055) but it always appears offline on the web interface to trace.

how can i fix this problem?

Project is called Traccar, not Traccer.

As for the problem, have you checked status screen in the app?