Possible cause 6 of 6:
The Wi-Fi access point is using a hidden SSID
If the Wi-Fi network is using a hidden SSID, you won't find the Wi-Fi network when searching for available Wi-Fi networks. You can only connect to a Wi-Fi network using a hidden SSID if you know the correct SSID.
What you need to do
Key in SSID and security settings manually on your mobile phone.
Did this solve the problem?