Mango doesn't not require an internet connection to be used so I'm not sure what the issues is.

Are you using a Modbus Serial or IP data source? The modbus serial has a selection for ASCII or RTU under the data source create page. Both serial and IP have a character encoding option for the data points but it depends on the modbus data type drop down list.

If you want to connect to Mango Automation from an unlimited number of IP addresses then you'll need to purchase a commercial license. To do this, click on the "Update your licenses now" button on the modules page.