Marco Polo App Deletion

The Marco Polo app can only be accessed with a verification code sent to your cell phone number. You have to enter this code to access your account. I changed my phone number and cannot access my account that is full of video chats I need deleted. I can't delete the app without my old phone number available to me. The app developers said they cannot help and if someone gets my new number they will have access to all of my conversations on Marco Polo.

What can I do to delete my account on an app that was downloaded to my phone if I can no longer access my old number?