We have checked your query and SfDataGrid will creates the DataRow in RowGenerator only for the rows currently in view. Hence you are not able to get the DetailsViewDataGrid which is in out of view and also you have to expand the corresponding parent record if it is not already expanded. You can bring the specifiedDetailsViewDataGrid into view, by usingDetailsViewManger.BringIntoView and ScrollInView method.
We have prepared the sample for this and please find the details below.
// parent DataGrid row index
int parentRowIndex = 25;
var record = dataGrid.View.Records[dataGrid.ResolveToRecordIndex(parentRowIndex)];
// MasterDetails View record
var parentindex = dataGrid.ResolveToRowIndex(record);
// Bring into View the MasterDetailsView Item
dataGrid.ScrollInView(new RowColumnIndex(parentindex, dataGrid.GetFirstColumnIndex()));