Hi Pastieb,
If you have a range of static IP's you are not able to assign one of them to the router, but what you can do is manually configure the router so that you can assign one of them to the VPN server then open the ports to the server for that public IP.
Another option is to downgrade to a single static IP, then reboot the router we will then auto assign the single static IP to the router, all you then need to do is open the ports to the VPN server.
I hope this helps
Markp