link to ISP network via another ISP
link to ISP network via another ISP
The VPN server must be forwarded through a port so it operates behind one NAT. This is your "free" (or almost free) method. Beyond that, you could deploy Pritanl on services like AWS, Google Cloud, Microsoft Azure, Oracle Cloud, etc., and connect the servers to that remote VPN. However, depending on your intended usage, this would incur associated costs.