I follow the documentation, tutorials and start the project in standalone (with Steam online). Steam overlay not working. Then I see there is a plugin for Steam (Online subsystem Steam), I activate it.
Restart the project , run the game in standalone and Steam overlay is working.
Not only does it work, I even triggered one of the achievements.
I package the game, start it and Steam overlay doesn’t work (nor do any untriggered achievements).
I uploaded the packaged build on Steam and overlay works than, but achievements do not.
The solution to this particular problem :
make steam_appid.txt with your game number on the first line and put that file in some of the shipping build folders (as described in the online subsystem steam documentation). Then upload the build on Steam and this should work.