In sprintr, going to your project, then in the left-menu going to Stories, then at the right side of the screen clicking on the dropdown More:
There used to be an option “Completed sprints”. Alas, no more.
Just pray that the (currently in beta) Epic-view will contain the historic Sprints again.
Otherwise; Download out Bizzomate’s SprintrinYourApp module and if necessary extend it yourself with calls to https://docs.mendix.com/apidocs-mxsdk/apidocs/stories-api/#25-microflow-getsprints
Got a project myself too, if you let me know your contactdetails, I will invite you.