It sounds like you may have lost your chat history when you upgraded your iPhone. If you didn't have a backup of your chat history, then it's likely that it's gone for good.
You've tried contacting WhatsApp support and using third-party recovery tools, but neither of those have worked. At this point, there's not much else you can do.
I'm sorry to hear that you lost your chat history. I hope you're able to find a way to recover it.