First of all: you can't prove that deleting posts is not one of the customer service jobs. His company will say that deleting posts is also the job of customer service.

Secondly, the employer agrees to hire you, but they can say that you can't adapt to this job and you can't take this job after training, so they will change your position, which is legal. Then you still can't do it. It is not their responsibility to ask for resignation.

Finally, you said that they agreed to subsidize pre-job training. Do you have any proof? If he refuses to give it to you, you can only seek labor arbitration. If not, you can also apply for labor arbitration. Two weeks' money. I'm afraid you will have to complain that you have spent a lot of money by then. Do you know China's national conditions?