RV50 NAT/Private LAN



May be a dumb question. The modems are working fine with our device using the Private LAN either thru port forwarding or a DMZ but we have an issue with one software application and ONVIF behind the NAT. Mid-stream they switch to the private IP when they are already talking to the public IP causing communication to stop.
If only one device is on the other side of the modem is there any way to set the device up with the static public IP(s)assigned to the modem instead of using the NAT.