I need help in visualising how to setup our VPN box when we move to our new network with Telstra.

We have a safe@office 500P, which has a public IP and a private IP of 192.168.19.2.

It is physically connected to our router, which has 4 different interfaces, one being 192.168.19.1. On the VPN box, we have a static route to forward everything to 192.168.19.1 which is the router, and from there it works out where to go.

Now, we are moving to a Telstra WAN and things are setup a little differently. Our head office router has only 3 interfaces- 1 is for the link to the switch that has the fibre connection (so our route to the internet and other branches), 1 is for our 10.10.20.x network and one is for the local branch network.

I really have no idea how to set this up as with the new setup, we will not have a port for it to plug into on the router. Could I just plug it into the 10.10.20.x network? Would I have to give it a public IP or can we just forward through the ports that it would use?

Another suggestion was to VLAN our switch into two networks- one for the 10.10.20.x network and one for the network the VPN currently sits on (192.168.19.x), and setup the router to trunk between the port and the switch. Not sure how to do this.

Sorry VPN's are definitely not my strong suit. Any advice appreciated!