A GST refund application for inverted duty structure must be filed within two years from the due date of the GSTR-3B return for the relevant tax period. If your application was rejected despite being filed within this window, the formal remedy is to file an appeal in Form GST APL-01 within three months of the rejection order. You may also seek rectification under Section 161 if the rejection resulted from a clear clerical or factual error regarding the dates.