If you bought DLC on Steam, you must play the Steam version, not the Itch version. The Itch and Steam version are two completely different games and save files. The Itch version doesn't recognize any DLC.
Can you join the Discord to get further help? I'd very much like to find out what's wrong then if you're playing the Steam version and bought the DLC, but it's not unlocking for you. Link to the Discord here, just say you need help in the tech channel.