Are you sure that you can connect to the Homehub. Is the wifi on the Lexus not a "wifi hotspot" which means you have a three G sim card in the car that allows you to connect your phone etc to the Internet.
See link about it
The range of your homehub is only a few meters. Once you got out the driveway you would loose connection so I don't see how it can work through your Homehub or what use it would be except if you were within range of a hotspot.
Can you either post a link to the manual or give the details of the vehicle etc so I can have a look.
I take it you are using the wireless passkey that is on the homehub label when you try to connect.
I have a similar problem. My Ford Focus will detect my BT Home Hub 5, but when I enter the password from off the back of the hub it is not recognised.