how are the two camera's connected by the coax line?
Do you have a 8p8c ethernet switch between the cameras and the coax+poe converter?

My read of that product page is that the converter transforms that coax link into a point-to-point link - i.e. one device at either end. if you need more devices at either end you need switches (or hubs) on those ends.
(after reading the Q&A, yes: you will need ethernet switches on both ends, as the device creates a single pt-2-pt link).
This addition might work, but you'd need to test: https://www.amazon.com/Linovision-Passive-Port-Extender-Splitter/dp/B07HSN5YSK