how can I find an unknown IP address

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Do you need the current configuration? If not, performing a factory default (assuming this box has a factory-reset procedure) might be the easiest solution.

If that is not possible or feasible, you might want to hook it to your laptop and start capturing it's traffic in the hope it will do an arp for it's gateway or try to do DNS or something else.

If that does not work, you might be able to use some nmap fu to try to determine it's IP address.