QGIS Convert MTM coordinate reference system

Question

I have X,Y coordinates that imported from csv file into QGIS and assigned WGS84 to the layer, when I open the attribute table and select Geometery $X it just shows the state X value and doesn't convert to lat/lng. Below is value of X,Y trying to convert.

294556.96,5039387.933

GEOGCS["GCS_WGS_1984",
 DATUM["D_WGS_1984",
 SPHEROID["WGS_1984",6378137.0,298.257223563]],
 PRIMEM["Greenwich",0.0],
 UNIT["Degree",0.0174532925199433]]
PROJCS["NAD_1983_MTM_8",
 GEOGCS["GCS_North_American_1983",
 DATUM["D_North_American_1983",
 SPHEROID["GRS_1980",6378137.0,298.257222101]],
 PRIMEM["Greenwich",0.0],
 UNIT["Degree",0.0174532925199433]],
 PROJECTION["Transverse_Mercator"],
 PARAMETER["False_Easting",304800.0],
 PARAMETER["False_Northing",0.0],
 PARAMETER["Central_Meridian",-73.5],
 PARAMETER["Scale_Factor",0.9999],
 PARAMETER["Latitude_Of_Origin",0.0],
 UNIT["Meter",1.0]]

Answer 1

Assigning a CRS does NOT reproject the coordinates.

Assuming X and Y are in the Canadian CRS you gave, you have to assign that to the CSV file. EPSG:32188 should be the right one.

Then use Save As ... to a different filename and EPSG:4326 as new CRS.