You should make the payment only after your paper has been accepted. From your question, it does not seem that you have received a decision letter from the journal yet. Please don’t make any payment before you receive a formal acceptance from the journal.
Possibly, the journal is not replying to your emails because they haven’t yet made the decision and don’t want you to make the payment right away. It does not make sense to withdraw your paper from such a reputed journal at this point; if you submit it to another journal, it will take several more months before they arrive at a decision. I think it would be best to wait till you receive the decision letter. Meanwhile, you can write to the journal, politely asking by when you can expect to receive the decision.