@audiobahn
If a device is accessible from other devices within the same subnet, but not from the VPN or other network segments it should be accessible from outside with NAT though, because this way the packets get a source IP from its own subnet.
However, in most cases it is the firewall on the respective device itself, which is simply blocking outside access. So the NAT is a hack and not recommended. You should better configure the devices firewalls accordingly.
There are only rare dumb devices, which have no possibility to configure a gateway, where NAT is a good workaround.