Topping up the portable wi-fi modem meant having to purchase a new sim card for it, however the Apple TV box isn't recognising the Optus service since the previous passcode was thrown out with the old sim card.
Any suggestions, anyone?
The passcode the Apple TV is looking for belongs to the modem, not the simcard. On the sticker under the battery, it should have a Wifi Access Point username and password which are the defaults. This is the password you will need to enter in your Apple TV, changing the simcard should not have changed anything. Did you activate the new Simcard with Optus?