Is there a way I can download bandwidth usage data details in a text file?
No at this moment. You can post your feature request at https://forum.peplink.com/forums/4-Feature-Requests.
On InControl2, go to Device Level (By click on any device name) -> Reports -> Bandwidth Reports, select hourly or daily or monthly, at bottom of each page, you will see a “CSV Download URL”, click on the link then you can copy the full link, you may use that link to download a CSV format of the bandwidth report.
Hope that helps.