Is the wage earned from working as an ATM supervisor for an interest-based banking institution considered unlawful (haram)?

There is no harm in a person working in something where they do not directly engage in what is forbidden, nor do they assist in it with intentional or direct aid. This is like supervising an ATM that may be used by customers of a Riba-based bank and others, and the transaction may be permissible or forbidden. However, your work does not involve direct engagement in the forbidden or intentional assistance in it. The Assembly of Muslim Jurists of America has stipulated that indirect or unintentional assistance in sin and transgression is not forbidden, such as selling something that can be used for both permissible and forbidden purposes without the intention of aiding in the forbidden. Nevertheless, it is preferable to seek work that has no connection to Riba-based banks, for whoever abandons something for the sake of Allah, Allah will compensate him with something better.