Use network socket (connect receiver)

You need to check where the cables of the sockets from the living room come out in the HAR, it is probably a so-called patch panel. Usually, the sockets in the patch panel and those in the living room are numbered sequentially. So if you take socket 3, you can feed the signal from socket 3 into the receiver using a LAN cable.

You then connect socket 3 from the patch panel with another cable to one of the 3 or 4 yellow "LAN" ports on the Fritzbox.

There are specialist electricians who wire LAN double sockets from the office to LAN double sockets in the living room. Additionally, a LAN cable from the utility room to the office. Rolled up behind the built-in TAE socket. Seen like this at the neighbor’s.

@TE Somehow first figure out which socket is wired where. If in doubt, ask the person who put together the cable mess.