I’ll try to add some food for thought: customer service agents and their managers do not know everything. In fact, most of the times, they do not actually know how the products work. They just know what they’re told and they’re only told what they ask. But if they don’t know what to ask, then they can get caught in a situation where they just default to blaming you or someone else so they can close the ticket.

Your best bet is to get in contact with someone who is in sales who can escalate this to an engineer or other more technical person. One way to do this may be to declare that you will tell your customers AMEX is no longer acceptable unless they help you resolve the issue. This may seem harsh but I bet it would work at AMEX and not a VISA or Mastercard