Well, if you use copper ethernet up to 1 Gbps there is no need to use the SFP cage at all, as there is a WAN port already that offers 1 Gbps copper ethernet (and you can only ever use that copper port or a SFP module, not both at the same time). Sure the official module will work and offer ethernet speeds up to 2.5 Gbps (the module is rated for up to 10 Gbps, but that omnia’s SFP cage/port is limited to 2.5 Gbps).
But a copper port will not allow a glas-fiber cable connector, you need some sort of media converter. If your ISP uses a point to point topology (often also called active optical network or AON) you can buy cheap media converter the convert ethernet over glas-fiber to ethernet over copper, but if your ISP uses a point to multipoint topology (using passive splitter, often called passive optical network, or PON)) then the “media-converter” needs more smarts, these then are often called optical network termination units (or ONT, but some ISPs also call a media converter for AON by the same name). E.g. GPON requires a quite sophisticated ONT and this ONT needs to be accepted by your ISPs GPON system.
What exactly is your hardware question?
If this is GPON they are using, you need to ask the for the list of devices they support, maybe this includes an SFP module (and maybe that is even generally compatible with Linux and specificially with the omnia’s SFP-port).
If your ISP already offers a “dumb” ONT that really has no router functionality then just using that ONT’s “RJ45” port to connect to the omnia’s “RJ45” wan port would be the simplest way forward.
EDIT: typo fix and correctin 2.5 Gbps instead of 25 Gbps