Hi Kostas,
Could you check again and see if it is appearing in the dashboard now? I suspect you may have run into the issue discussed in this thread where our Pi & Arduino server was causing devices to show offline while they were otherwise actually online, rather than any configuration issue on your end with your network or the ESP itself.
We’re believe that server is stable at the time of this writing, and will be replacing it with a new one that is in development to ultimately prevent the issue from happening again.