Hi Arash,
Unfortunately the ALARM configuration for MODBUS Master has the limitation of only being able to configure the read of a single 16 bit register, so if you are reading a 32bit float value, it wont get the correct data so it wont work properly.
For workaround, i'd suggest you using the "Data to server" service and forward all Modbus data you want to a server that can analyze the received data and if it is below/high than a determined value, you can use the SMS Gateway functionality of the router to send sms messages when your application running on your server tells the router to do it. I believe that using python you can do that without much trouble.
Best regards.