No box found
Make sure this phone is on the same Wi-Fi as the box, not on mobile data, and that the box is plugged in and set up.
A box checks in when it starts and every 15 minutes after that.
A VPN, or iCloud Private Relay on an iPhone (Settings › your name › iCloud › Private Relay), hides which network this phone is on: turn it off and try again.
Try againA box's own link (in its Settings) works with either on.
This phone reached the finder from 216.73.217.154.