The setup is refreshingly simple, I added an SFP modul (FLEXOPTIX S.B1312.10.XDL) to the Cisco SG300-10 switch I used before, that was all. With my new provider iWay it was not even necessary to configure a specific VLAN.
On there website they say you need a
Simplex TX 1310nm/RX 1490nm BiBi SFP Module.
The SFP module is connected to port 9 on the switch, port 10 goes to my OPNsense router. Ports 9 and 10 are on the same VLAN, just via access ports to separate the traffic. Port 8 gets internet from the router and distributes it to port 4, my office switch.