Some additional information
Customer Network IP can not be changed
Yes it can but custom is not going to do this.
The "Item" a web enabled industrial SCADA circuit board
(IP to be considered unchangeable)
Yes, its changeable if you can plug into it RS485 and have the correct software.
But its 2000 miles away and software is propriatary and not sendable to customer
and don't want to incure the cost of sending ethernet to 485 hardware and talking to it remotely.
As to pubic / priviate
Its a standard office, a number of computer on a networks and a router device to give them internet access.
My thoughts were a bridge or router.
Just wanted to get a opponion from someone who has connect two LAN connections that might be similar to my situation.