Hi,
as described in the link below you must first got to LUCI and
then set up the port to your LAN zone, and then you may be
able to use it there, but as I am informed you can only use
the WAN marked port or the SFP port and not both at
the same time.
SFP port usage
Edit:
From another forum thread about the same question
Turris Omnia has three physical Ethernet ports. One of them (eth2 ) is dedicated as WAN and connected directly to WAN port. That can be either metallic or SFP. If you plug in SFP module, the metallic port will get disconnected. If you unplug the module, metallic port will be functional again. In Turris OS 3.X this can be done at runtime, on newer version changing the configuration requires reboot.