It's probably the NAT configuration. It would be useful to see that. On the router itself, packets go directly from the connected interface and don't go through NAT, which explains your pinging.

The SFP can't tell what fiber type is in use, all it knows is that it receives a good signal or not.Every SFP has a range that depends on the fiber type and OM-4 generally has a longer range, so if you're within that limit you should be ok. How long ...