I'd like a cheat code as well, but I don't think there is a cheat within the game itself. You'll have to edit your save or modify your game's data in some way afaik.
Luckily, Majalis included hints to figure out what scene(s) your missing in the "Progress" menu (although if you're using the public build, it will sometimes include scenes that can only be done in the patron exclusive Story Mode)