If your gateway’s MAC address starts with 04:CF:8C or 7C:49:EB, there is a good chance that the required port 9898 is closed on your gateway (you can check it with the Nmap utility, using the command sudo nmap -sU {gateway_ip} -p 9898). To fix that issue, you need to do these steps:
Find a specific screw bit (like a fork) to open the gateway case.
Find a USB-UART cable/module and connect it to your computer.