Normally your best bet unless there is a local Repair to you is to try and source a defective camera on Ebay seeing there isn't many people selling camera PCBs.. If I was rich I would do it my self. But I have to much money in my own gear and do my own Repairs. Then not sure what type of camera your PTZ board is going to but the 100 or so Ip cameras I have repaired over the years some much easier then others.. What is the Model of PTZ you are working with and how do you know that you need an RJ45 board? Lot of people don't know that PTZs are a complex system with many different PCBs from POE/Power, Main control PCB and Camera PCB.
If your camera has a BNC connector and connecting the camera to 12 or 24v power depending on the camera and you get video out of the BNC don't always have to point to bad Ethernet Board..
I am sure you have but have you tried a different Ethernet cable? Or Port on Switch/Router/NVR?