Hello,
I'am not sure about using RPC, since I did not use Thingsboard much. However, it appears that it is possible to subscribe and publish to a third-party MQTT broker from Thingsboard. You can find a Thingsboard tutorial available here, which uses the HiveMQ broker as an example. I cannot really help much with the implementation from the thingsboard side of things, so I suggest checking other MQTT articles on the Thingsboard website. Once you can publish and subscribe to the HiveMQ broker from Thingsboard, you can configure the RUT955.
The RUT955 can work with HiveMQ Broker. To read from and write to Modbus devices, including the RUT955 itself (when the RUT955 has Modbus TCP client enabled), you can utilize the Modbus MQTT Gateway functionality. This allows you to publish to a specific topic on HiveMQ, and the RUT955 will respond accordingly. You can refer to our wiki page here for information on how to use the Modbus MQTT Gateway. If you prefer to use your own broker instance in HiveMQ, I recommend reviewing this thread here, which provides instructions on how to do so.
Please, let me know if you will have any issues with Modbus MQTT gateway. Also, it would be greatly appreciated if you could share your results in case you successfully implement it.
Kind Regards,
Andzej