Based on your itinerary, it seems that you should be able to get funds while at Iguazu. You can get money from an ATM at the Melia, ask the front desk to make change, or perhaps you can ask your taxi driver to stop at a cambio in Puerto Iguazu once you've crossed the border into Argentina. In a worst case scenario, there are ATMs at AEP and there should still be Banco de la Nacion window at AEP where they will exchange funds for you. Just get a small amount for the cab fare in either event until you're able to get to a proper cambio to exchange at the blue rate.

Once you're at the Hyatt, you can ask the front desk if they have a recommendation for where to find a good cambio. When I've stayed at the Hyatt, I've usually gone down the street (Posadas) to a place called Cambio Posadas - it's a couple of blocks over and they're very nice and professional. It's a small place but you go in, take a number and when they call your number, they actually usher you into another room to do the exchange. It's very clean and safe and in a nice neighborhood. Here's where you can find it on the map: https://www.google.com/maps/@-34.587...8192?entry=ttu