Adding new grouping to a report
There is no way to do this in the User Interface, you would have to develop a little extension that customizes the Contribution Summary Report by adding the Payment Instrument as an additional grouping. You can find some guidelines here: https://docs.civicrm.org/dev/en/latest/framework/civireport/

Or log a request at Gitlab for an improvement to the core report. You can do that here: https://lab.civicrm.org/dev/core/issues