Keep in mind that passwords are case sensitive.
The preset DD-WRT FlashRouter WiFi password is Wireless#[First 5 characters of your router MAC]. The router MAC can be found on the bottom of your router. For e.g. if your MAC is 1459C07E70A8 then your WiFi password is Wireless#1459C
Verify wireless password
Connect an ethernet cable from a numbered port on the FlashRouter to a computer and access the FlashRouter’s settings.
Navigate to Wireless > Wireless Security
Check the Unmask box and note the wireless password that is entered.
Verify the wireless settings
Make sure that you have WPA2 Personal / AES set as the security settings for all bands. For some router models make sure the CCMP-128 (AES) box is checked for all bands.
Forget network on your device
Wireless devices usually offer a “forget” network feature to remove a wireless network from the saved information of the wireless adapter. If you have updated your WiFi password and are trying to connect on a device that previously used the old WiFi password then removing the wireless network and then trying to connect fresh can help with this issue.