I'm not sure that you can in your case. The async ports are typically used to communicate with other routers also using an async port via a dial modem or similar (using, say, PPP). Of course, in the case of the AUX port, you can also interface to router as a console port. I suspect your ATM machine implements some type of protocol that the router does not use. You can't just set up a raw async end-to-end pipe via the routers, as far as I am aware.
