Hello and thank you for contacting us, this is Martín, Tech Support Engineer for Teltonika Networks.
I don't understand the issue correctly. Are you trying to send and receive MQTT requests and responses?
The title of the question says Modbus Gateway, yet there is no mention of any Modbus configuration in the body of the question. Can you please elaborate more about what you are trying to do?
Also, can you please attach Backup and Troubleshoot files? You can find instructions on how to download Backup files here, and Troubleshoot files here.
Both files can be attached to the original question in the forum.
I remain attentive to any further replies to help you solve this issue.
Best regards.