strange port forward behavior

I have a client who is on Uverse internet at his business and AT&T recently replaced his gateway with an Arris NVG589. He has a VPN endpoint behind the gateway and it stopped working after the new gateway was installed. I noticed that the port forward that was previously configured was not there, so I set it up and it seemed to be working. They let me know that it works, but after a few minutes it stops for several minutes. I looked into it and it seems like this gateway port forwards destination IP can only be set by a drop down box named "needed by device". It seems to tie an IP to a mac address. When someone connects to the VPN, the gateway is associating the "device" with the IP address of the newly connected VPN computer. Somehow it is reading the VPN address as having the mac address of the server and it changes the port forward automatically, which breaks the VPN connection. I can see why this behavior *might* be useful to someone, but I just need to set a port forward to a static IP address and this new gateway doesn't let me do that. Seems like a pretty silly feature to be missing from a "business" gateway. For security reasons, I can't use IP passthrough "default server" mode (In testing, this does work, but it allows other traffic through that I don't want.). Any way to enable "dumb" port forwarding on this gateway, or can we get it switched out (again) to a gateway that lets me do this?