Hi Nikospps, welcome to the forum!
The Modbus scan tool is only to make identifying slave IDs somewhat easier or assessing which slave IDs are currently up. The tool simply attempts to read Holding Register 0, and if it succeeds it reports that slave ID as existing on the network.
You do not need to purchase Mango in your use case.
You need to create data points that correspond to the data mapping in your external device. If it's a standard sensor / component / PLC it may have a modbus map in the manual for a device that explains register range, data type, register offset and possibly scale factors that you can use to create points. Try using the "Point locator test" tool on the same page as the scan and using that to create a data point.
If your device has hundreds of registers preconfigured in that modbus map, know the free license is limited to 300 points and non-commercial use so you may need to be choosy about which points you are collecting data on.