Did you make sure to set Edit → Input Devices → NetworkTablet to Screen? Other devices like a mouse can’t be used together.
Computer not detecting input signals from tablet
I’ve followed every instructions that you gave to others who had my same problem, like disabling firewall, uninstalling and re-installing the the app from the tablet and i tried to update the driver-uninput folder by replacing the one in /home/ with the most recent version that i downloaded from the GitHub repository (https://github.com/rfc2822/GfxTablet). I also double-checked if the IP address i put in the app was correct and the two devices are connected to the same network.
Thank you in advance.
Please check the network traffic with tcpdump or Wireshark.