Thanks for the reply. Actually, what we want is something else.
We have a solar controller which has RS232 output. When I connect that to a PC I can read it. I want to read it via the Teltonika, ultimately using RS232 Server over TCP/IP mode, but provisionally any mode, including console.
What I've tried is: Connecting the Solar Controller via RS232 to the Teltonika, enabling Server > Console mode and using cat /dev/ttyS0 on the CLI which works from a PC, but doesn't work on the Teltonika.
We've also experimented with TCP/IP mode, still no success.