Do you have some document saying that you bought/own the condo unit as leasehold or foreign freehold?
If yes, talk to the cops, show them proof of ownership, and tell them you are illegally being denied access to your property and ask them to intervene.
If that doesn’t work, ask a Thailand lawyer to write a letter to the developer, warning of legal action unless they give you access to your property within two weeks. Should only cost a few thousand baht for the letter.
if they are conning you they are probably conning others. Create a group on here or on facebook etc for conned owners of that project. Perhaps as a last resort you can take the developer to court as a group.