Note that the use of WEP is not recommended for compatibility, reliability, performance, and security reasons; WEP is insecure and functionally obsolete. 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.
I use WEP 128 and enter the first hexadecimal key as the password and the router is setup to use the first key.