Is the router using an ASCII or a Hexadecimal WEP key, because some network devices don't like ASCII keys?
You can tell which type is used by the number of characters:
64-bit WEP uses 5 ASCII or 10 Hexadecimal characters
128-bit WEP uses 13 ASCII or 26 Hexadecimal characters.
[ASCII characters are basically the numbers, letters and symbols on the keyboard; Hexadecimal characters are "0" to "9" and "a" to "f" only, a total of 16 available characters.]