Diagnosis
The Wi-Fi module in the faucet is probably stuck or lost its network settings. It's a finicky 2.4GHz-only connection.
How to Fix It
Hard reset the Wi-Fi. On the faucet panel, press and hold the 'Flush' button for 10 seconds until the Wi-Fi icon starts flashing rapidly. This clears the old network data. Then, go through the app setup again from scratch, making sure your phone is on a 2.4GHz network (not 5GHz).
If That Didn't Work
The module itself might have crashed. Unplug the transformer powering the unit from the wall for 5 minutes. This fully reboots the electronics. Plug it back in and try the setup process once more.
Heads Up
The faucet's Wi-Fi signal is pretty weak. If your router is far away or there are lots of walls in between, it may never connect reliably. You might need a Wi-Fi extender.