Q:

LAN only works if connected directly to the router

I have this setup:
A router in my living room that "feeds" all internet plugs. In my office the LAN connection works correctly with my PC and PS4 but my Raspi 4 does not detect any signal. However, if I connect the same Raspi 4 directly to the router through LAN, it works fine. In all experiences I have used the same cables.
What could be the source of the problem?
EDIT:
Not working ethtool eth0 log:
Settings for eth0:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
                                             1000baseT/Half 1000baseT/Full
        Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 1000Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: no

Working ethtool eth0 log:
Settings for eth0:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
                                             1000baseT/Half 1000baseT/Full
        Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 1000Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes

A:

The Pi4 (and Pi3B+) has a Gigabit Ethernet interface.
To work this requires 4 pair high quality cabling.
Earlier models used 100baseT/Full which works on only 2 pair.
Unfortunately, while the interface SHOULD negotiate a compatible mode the autonegotiation operates on 2 pair, so if both ends support Gigabit Ethernet this will be selected, even though the cabling does not support it.
You can downscale the link speed with ethtool commands. I have done this in the past, and the following should work, but I am not exactly sure of the syntax and order of commands.
sudo ethtool -s eth0 autoneg off
sudo ethtool -s eth0 speed 100

This should allow your Ethernet to work at a theoretically slower 100MBPS speed, but it is doubtful that you would notice.

