Apple designed the AirPort routers for use with standard DSL and cable type connections. We've seen a number of issues on the forum from users who have a fiber connection, but I don't have fiber, and I don't think LaPastenague does either.
So, we can't really offer any specific advice on troubleshooting a fiber connection based on our own experience. I don't like to guess at thiings, and will not answer questions If I don't have similar type equipment or I haven't done it myself.....so I know where the trouble spots might be.
All that I can suggest is the same standard things that the user has probably already tried......
Reset the AirPort Express
Try a different Ethernet cable
Try another AirPort Express
If a second sample performs the same as the first, then there is likely compatibility issue that the AirPort is having with the fiber connection. Unfortunately, not much that you can do about that. An Apple router might not be the right product in this case.