This is a common problem, as NAT tends to interfere with many VPN solutions. If you need the VPN to be behind the firewall rather than in parallel with it, using a routable IP address is not only okay, but likely the only way it will work. However, with many firewalls, you can have a separate set of firewall rules for the interface the VPN is connected to. Thus you can setup your firewall to block all ports and protocols other than what is needed by the VPN. That will provide the maximum level of protection.
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