DIAGNOSIS This is almost always a Wi-Fi setup or signal issue, not a grill hardware fault.
FIX On the grill, hold the Wi-Fi button for 5 seconds until the light blinks rapidly. On your phone, forget any existing "Ninja_Grill_XXXX" network. In the app, re-run setup. Ensure your phone's Wi-Fi is on 2.4 GHz band. Place the grill within 15 feet of the router during setup.
IF THAT DIDN'T WORK The grill's Wi-Fi module may have frozen. Unplug the grill from power for 10 minutes. This performs a hard reset on the module. Plug back in and try setup again.
HEADS UP The grill cannot connect to 5GHz networks or networks with special login portals (like apartments, dorms). It needs a standard 2.4GHz home network password.