I have a shapefile, currently uploaded into QGIS of some vectors. I took the coordinates of the middle of these vectors in order to find their x and y coordinates. These coordinates are for instance: X 189372,586045238 and Y 519791,329052381. However, I need the long lat coordinates in order to link this data to other databases.

Already tried selecting EPSG:4326 as CRS, the tip in:
https://gis.stackexchange.com/questions/64535/converting-x-y-coordinates-to-longitude-latitude-using-qgis
However, the coordinates do not change into the coordinates I want as a result.

Also tried the programming solution (Python) mentioned in:
https://gis.stackexchange.com/questions/64535/converting-x-y-coordinates-to-longitude-latitude-using-qgis
However this code gives a lot of errors and results in nothing for me.

I think I'm missing something. What is it?