It's in the Dashboard. Access is by a swipe gesture, not a static button.
Swipe down from the top of the printer's display. This opens the control panel Dashboard. Look for the gear (settings) icon. Tap it. Select Network Setup. The Restore Network Settings option is there.
Use this to restart the Wi-Fi setup mode if the blue wireless light isn't blinking. After selecting it and confirming, wait one minute before retrying in the HP Smart app.