You can indeed have the internal transaction number show up in the Transaction Detail Report.  It is actually called the Transaction ID.  The trick is to use the New Enhanced Experience (or Modern View) version of the report.  This version is available in QuickBooks Online Advanced.  It allows for a whole lot more customization of the report including adding many more columns than what are available in the Classic View Transaction Detail Report.  See the image I have attached that shows how to add the Transaction ID column to the report. 

If this has helped you and you do not already have QuickBooks Online Advanced, we are an Intuit Authorized Reseller and can provide you with a discounted upgrade to QuickBooks Online Advanced.  Feel free to reach out to us for details.