How to configure NVG510 to allow internet access to an NVR on the LAN.

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About a month ago I installed an NVR on the existing LAN of a local lumber company. The NVR is just a PC running Vivotek server and client software. The existing LAN has a connection to the internet through AT&T U-verse,  the modem is the Motorola 2210-02. The modem is then connected to a Snapgear VPN router. The lumber company has a third party marketing firm that maintains their VPN, so I called their tech support people and they helped me set up the port fowarding on the router to the NVR. The port fowarding never worked. Right now the public IP is assigned to the router and not the 2210. The VPN tech support people informed me that they have run into this particular problem with other customers, meaning that the port fowarding has not worked for several other customers that needed to attain internet access to an NVR. The VPN tech support people suggested that I contact AT&T, so I did and this is what they told me.

AT&T told me that port fowarding would have never worked because of the model of modem and also due to way AT&T has control over the current public IP.  AT&T said the only way that I could access the NVR from the internet was with the NVG 510. It is at this point where I need help. I have the NVG510 and am ready to swap out modems, but I have some questions and concerns.

All the PC's on the existing LAN have been assigned private addresses manually, there is no device offering DHCP.

They all have 192.168.3....... on the network. My concern is that AT&T told me that the NVG510 would have to be configured to offer public addresses which would seem to me to cause problems with the existing network configuration. Obviously I would like things to stay exactly the same on the LAN. How can there be public and private addresses on the LAN and everything work like it should?

Current IP address config:

2210 modem - 192.168.7.254

VPN router - 192.168.3.254, the public IP is also assigned to this router

NVR - 192.168.3.206

In summary, how can I access the NVR from the internet without affecting the current network configuration?

The lumber company is fully dependent on the VPN and I don't want to screw up any existing network configurations designed by their marketing firm.

Thanks in advance.
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