Hi Daniel,
Could you Please try using inserted at the place where you need the line break and set the WrapText property to true.
[C#]
// New Line
sheet.Range["A1"].Text = "HelloWorld";
sheet.Range["A1"].CellStyle.WrapText= true;
Here is the sample for your reference:
WrapText.zip
Please take a look at the sample above and let me know if you have any other questions.
Best Regards,
Melba