Using Modbus TCP server on Arduino
The ModbusIP library expects from you that you supply the value of the register. The AirSensor library gives you that value.

Set the register value to Hreg:

mb.Hreg(SENSOR_ISTS, airSensor.getTemperature());


I tested your sketch without the sensor library and it is working. Client was my java test client I use to test access to Modbus TCP registers of my photovoltaic system.

Make sure that the client calls "0x03 - Read Holding Registers" and test address 100 and 101 because some modbus clients offsets are 1-based.