Are yu doing this as specified in a sublink in the link I provided:
However, if you must support legacy WEP devices and you have a newer (802.11n) Wi-Fi router, you may be able to select the WEP Transitional Security Network (WEP TSN) security mode. This mode will allow legacy WEP clients to join your network with WEP encryption while allowing newer devices to use more modern and secure encryption modes, such as WPA TKIP or WPA2 AES. If WEP TSN mode is not supported, then WEP128 with Shared Authentication should be used (with a single WEP key in key index 1). For compatibility reasons, WEP128 networks should use 13-character ASCII passwords.
For reference, "None" or unsecured mode, provides no authentication or encryption. If you use this security mode, anyone will be able to join your Wi-Fi network, use your Internet connection, or access any shared resource on your network. In addition, anyone will be able to read any traffic you send over the network. For these reasons, this security mode is not recommended.
I aadded the bold since that may be your problem
Above from:
iOS: Recommended settings for Wi-Fi routers and access points