@PlusSP
I am sorry, but the description of your existing and intended connections is confusing.
1). You can not use eth1 of FB3 for anything else but upstream internet connection (WAN). You should leave this connection as-is, do not change the wiring there.
2). You can connect your LAN devices through any of the remaining eth ports of the FB3. This part of the LAN can also be expanded using switches (if you run out of eth ports).
3). How do you know if the bedroom and main room sockets are connected? Very likely they are not. If you want to use the socket in the bedroom you have to know where is its other end - it is certainly not in the main room (the socket in the main room is connected to the internet, not to your bedroom).
4). I suspect (speculation) that there is a box in your apartment somewhere, with both cables (from the bedroom and from the main room) terminated in sockets. The cable coming from the main room is either connected to ONT there, or (more likely, because this PoE injector is there to power something outside of your apartment with your electricity - sound like the ONT located closer to the fiber box located outside of your apartment needs this power) just extended to the externally located ONT. The cable from the bedroom is likely simply not connected to anything, and terminates in a socket (in that box). Now, the intention was to have the router in that box, and then all cables form the box could be used: external for WAN (to FB eth1), bedroom and main room eth2-4. Why was this not done? Most likely not enough room for FB in the box. There is very little you could without rewiring (second cable form the main room to the box would be needed, where it could be joined with the bedroom cable), unless you could actually stick FB in this box (you have to find it first).
