You are right Justin. That's why I'm asking for a Nodered hosted on Teltonika Router or gateway.
Modbus data to sender by MQTTs is limited if you can't modify the server application on Cloud. For example, on JSON segment, %t return linux timestamp without ms.
JSON segment could only send the SlaveID Number, not his name. And data are only described as an array of register, based on address...
If you monitor hundred of devices, it's difficult to rename all values.
If any case is different for different usecase, it will be better to integrate Nodered to adapt the data formatting, even if it's a vanilla version without ability to add complete node.
With it, you can answer to many usecases without modifying firmware.
Ps : using a little Pi isn't possible on industrial case, not compliant with harsh environnement.