Open your phone's settings and make sure you are connected to the correct Wi-Fi network.
These options may vary depending on your device.
Look for the Wi-Fi connection indicator at the top of your mobile phone screen.
If no indicator is displayed or no indicator bar is active, you may be outside the range of the Wi-Fi network:
Move closer to your FRITZ!Box, check whether the Wi-Fi connection is better and try again.
Switch off the FRITZ!Box's Wi-Fi and restart it. To restart it, press the "WLAN" button on the box for 2 seconds. Then try again.