Pattern Help: Passing Object from DAL to Contract. Two Classes, One Interface

I have many projects in my solution representing the different layers of the application. The Data Access Layer (DAL) has a model of the database in it and more importantly --for my issue-- a Plain Old Class Object (POCO). I want to send an instance of this POCO to an external requester via a WCF contract. As you know, I must define the Operations Contract and Data Contract at the contract layer. It is here were my problem lies, how do I declare the data contract and its data members when the POCO is situated in another layer?

I have tried defining an interface and have both classes implement it, but I come up against a problem when I am getting the objects from the database and then passing them through the contract, the contract does not know the object being passed to it - even though it shares an interface.

Anyway, hope that is clear (as mud!), and if anyone can advise me on a suitable solution I would be much obliged.

P.S. Using C# in VS2015