If you feel like the online dealer has cheated you the first thing you should do is try to take a screenshot of the game as proof that you have been cheated. Some online casinos allow you to replay the last hand, which will allow you to take a screenshot then. You could then call the help centre and explain what happened. A screenshot or some form of proof is important as then it cannot be denied and most online casinos require proof before they can consider the next step.