Hi Balu,
i checked popup windows are not blocked in the Safari settings. Further investigation on this issue, I found that the Rest API is generating excel using Open XML.
There are some sources that suggest Strict Open XML is not supported on IPAD
Also, I created excel file in .NET using Open XML with .NET Dataset object and this file also didn't open in iPAD. But with EPPlus library it was working fine. Something is problematic with OpenXML when using .Net Dataset object for excel export.