Hey there, I know this is an old reply but hopefully you will see it 
I first started by testing with Voxel's FW and there I was able to bring up the GPON SFP Module (Realtek DFP-34X-2C3) just fine and access it management UI but the problem with Voxel is vlan support on eth0 (SFP+ Port) which does not allow you like OpenWRT does, so I git cloned their repo and started digging until I found this that forces your SFP+ Port to works on 1000base-x
echo 1 > /sys/devices/platform/soc/soc:pcie-internal/pci0000:00/0000:00:00.0/force_1000_base_x
Then to make it persistent across reboots I added it to /etc/rc.local:
echo 1 > /sys/devices/platform/soc/soc:pcie-internal/pci0000:00/0000:00:00.0/force_1000_base_x
ifconfig eth0 down
ifconfig eth0 up
exit 0
Then just configured it normally and it have been working perfectly.
Hope this helps you and others