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 slecting EPSG:4326 as CRS, the tip in:
https://gis.stackexchange.com/questions/64535/converting-x-y-coordinates-to-longitude-latitude-using-qgis
However, the coördinates do not change into the coördinates 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?


EDIT:
 still can't track the coordinates to a separate column in the attribute table.

My QGIS currently is in EPSG:4326, and the data points are visible. The CRS which the data originally has is a custom CRS. However, when I swipe with my mouse over the coordinates, it shows coordinates of 51 north and 5 east. Having a map of the Netherlands indicates the custom CRS does know where it is compared to the map of the world.

I did all the things commented above.