How to secure the voip (vonage) router which is connected to the primary netgear router.?

I ve got a voip service called vonage which comes with a wireless router. This must be connected to Lan 1 of the Netgear router which provides internet so I can make phone calls through internet. The problem is that I do not know how to put a wep key to the vonage router(DLink) so that no one else can have access to my internet connection. Notice that the Netgear Router has an encrypted key and it comes with Virgin Broadband. However even if the first is encrypted the second is unsecured and anyone can have access.