@subpopculture 

I believe that you already tested adequately, by using, from port 2 of the main Deco, different cables to different devices, none of them working on port 2, each of them working on port 3.

I did some more testing since posting this, including a factory reset. After the factory reset, it was Port 3 that was not connecting instead of Port 2. This showed me that the issue is likely caused by software, not hardware. I was "fortunate" enough to have had an earlier issue that I raised with TP-Link support. As part of this, they installed a development build firmware on my X55 which allows for shell access to the device. So I connected to the shell and started looking at the bridge interface configuration, having a hunch that the issue is caused by the automatic interface assignments to the WAN and LAN bridges. It turns out this hunch was right. The device has two bridges: br-wan and br-lan. They were configured with eth1.3 and eth1.593 assigned to br-lan, and eth1.2 and eth1.4 assigned to br-wan. After a little trial and error, I figured out that eth1.2 was my ISP uplink interface, so I took eth1.4 out of br-wan and assigned it to br-lan. That "fixed" my "dead" Port 3.

root@X55:/# brctl show
bridge name     bridge id               STP enabled     interfaces
br-iptv         8000.000000000000       no
br-lan          7fff.74fece0bc97c       no              ath0
                                                        ath02.1
                                                        ath02.2
                                                        ath1
                                                        ath12.1
                                                        ath12.2
                                                        eth1.3
                                                        eth1.591
br-wan          8000.74fece0bc97d       no              eth1.4
                                                        eth1.2
root@X55:/# brctl delif br-wan eth1.2
root@X55:/# brctl addif br-lan eth1.2
root@X55:/# brctl show
bridge name     bridge id               STP enabled     interfaces
br-iptv         8000.000000000000       no
br-lan          7fff.74fece0bc97c       no              ath0
                                                        ath02.1
                                                        ath02.2
                                                        ath1
                                                        ath12.1
                                                        ath12.2
                                                        eth1.2
                                                        eth1.3
                                                        eth1.591
br-wan          8000.74fece0bc97d       no              eth1.4

I am skeptical this will survive a reboot of the device, but I have not tested that yet. I also don't know where in the init scripts or config files I can go to fix this permanently.

  @subpopculture 

Interesting!

By the way, 591 is the vlan id for the guest network.

  @yves_b Ah! That clears things up. I wasn't sure why there were 4 interfaces in these two bridges, considering the X55 only has 3 ports. This makes so much more sense now. If the naming assignment is consistent, then it goes from the bottom up.

Port 3 is eth1.2

Port 2 is eth1.3

Port 1 is eth1.4