The usual place to find the WEP key is underneath the router, possibly near to a barcode. Did your IP give you a password? Is this on the bottom of the router along with the WEP key or in a letter you may have received from your IP? If so did you then have to change it to something more personal? If you find the original password you could reset the router back to factory settings and it should revert back to the default password.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been