Media converter (eth/fiber) problem in communication
I think you have a speed mismatch issue.

Unlike most twisted pair ethernet equipment gigabit media converters are usually single speed devices. I'm not positive as to the reasoning but I think there were substantial changes on the fiber side that made supporting both systems in the same media converter difficult. This is a problem as the ethernet controller on Pi models up to and including the 3B is a 10/100 device while your media converters are gigabit devices.

I see four possible soloutions.


Use a USB gigabit ethernet adaptor on the Pi.
Use a gigabit switch between the Pi and media converter.
Replace the media converters with 100 Mbps models.
Replace the Pi with a "raspberry pi 3 model B+" which has a gigabit Ethernet controller.