DIAGNOSIS The Bluetooth/Wi-Fi module in these things can be finicky. The most common failure point is the initial handshake because the vacuum's wireless radio goes to sleep quickly.
FIX Put the vacuum right next to your phone and router. On the vacuum, press and hold the "Wi-Fi" button (usually near the power button) for 5 seconds until you hear a voice prompt or see a specific blinking light. Now open the Tineco app and follow the setup again immediately. Make sure your phone's Bluetooth is on—it uses BT for the initial handshake.
IF THAT DIDN'T WORK Forget the device in your app and in your phone's Bluetooth settings. Power cycle the vacuum (turn it off, wait 30 seconds, turn it on). Then repeat the setup process. Also, ensure your phone isn't connected to a VPN or using a "private" Wi-Fi address, which can block the local connection.
HEADS UP The app features are mostly for stats and reminders. You can't actually drive the vacuum with it. If you can't get it connected, the vacuum will still work fully manually, you just lose usage data and firmware updates.