You need to reconfigure the VPN client on your computer at home. It has to know the WAN address of your remote site. Your remote site is using NAT as is your home site in all likelihood. Let's say your LAN at work is 10.10.n.n and your home LAN is 192.168.n.n. When you wish to open a secure tunnel you use the 10.10.n.n address and your client opens the tunnel and routes between the subnets. It knows where to go because of the WAN address you assigned to that subnet. So all that needs to change is the WAN address in your VPN client. Depending on the VPN software you may have to regenerate keys.