With RUTOS firmware versions (00.07.00 and higher), the USSD command's output is stored in a slightly different file.
The file's name would be different for different device models.
In your case, the file's name would most likely be "ussd_1-1.4_1". I.e. "cat /tmp/ussd_1-1.4_1" command should show you the command's output.
In case this command would also not work, or if you would ever use different Teltonika devices/models, you can do the following:
- Execute USSD command with the same command: gsmctl -U "*100#"
- Use "ls /tmp" command to see all files in the /tmp directory. You should look for a file, which would start with "ussd" text. Alternatively, you can manually type "cat /tmp/ussd" in the command's field and then press the "tab" keyboard button, so that the SSH window could autocomplete the command.