Want to disable my wifi and add another unit

It does not look possible for the CG3000DV2 to be used solely as a cable modem.  But that should not stop you from running a VPN client on your home network.  You can set things up so that devices send traffic to the VPN client and it sends everything through the VPN tunnel.

But you should understand what you gain by using a VPN.  A VPN adds a layer of encryption on the traffic between your home network and the VPN provider.  This certainly will protect your traffic from your ISP's prying eyes.  But once the traffic arrives at the VPN provider, this encryption is removed before it continues its way out to the rest of the Internet, and it can again be snooped upon by prying eyes.  Of course, if the original traffic was encrypted, then you should be relatively safe, but then the value of VPN encryption is basically zero.  And it should go without saying that you can still download malware through a VPN.

Of course, VPNs have other benefits, such as shifting your apparent location but as far as protection of a home network is concerned, your router's firewall is your main line of defense from hackers as well as antivirus/antimalware programs on your computers.