DPI (dots per inch) in JPEG EXIF, JFIF or PNG pHYs tells a printer how to map pixels to paper. It does not change how many pixels exist. A 4000 × 3000 px image is 13.3 × 10 in at 300 DPI and 55.6 × 41.7 in at 72 DPI. The pixels are the same. Setting the tag from 72 to 300 without resampling does not add detail.
DPI-Pruefer
Echtes DPI-Tag und Druckgroessen. 72 auf 300 zu aendern erzeugt keine Details.