• Gateway with any firmware can be flashed to custom firmware via UART
• Gateway with any firmware can be updated to custom firmware via Telnet

Article about flashing in Russian language and Google Translate.

• USB TO TTL 5V 3.3V or this model - 3.3v important!
• PCB Clamp no need to solder! Size: 2.54mm and 3P or more
• Ethernet Connector if you want to solder it