I’m not sure how thingsboard does addressing, there are two different ways in Modbus land (wire addresses, and modbus addresses). See this for more information:
If you turn the debugging up to 9 on the Modbus node, capture and post the log here, then we can make sure Thingsboard is reading the address SIOT is expecting.
Tags are used mainly for tagging data in InfluxDB, so not needed in this case. The Modbus data will come from the Value field.