Creating a VPN without a registered IP f

We have an ISP that ONLY uses DHCP, so I use NAT on my Ascend Pipeline 75. I need to set up a VPN, but haven’t found out how without a registered IP on the router. Can’t change ISP’s at this time and they don’t give out IP addresses. Only need two addresses and InterNic won’t give them out in that increment. Can anyone help?