Crazy puzzle: partial ethernet connection - need ideas


So here is one that got me completely puzzled.

Here is the setup:

Building common security cameras (managed by another company)
Connected to a switch downstairs in the compound
Switch is connected with a 3 pair ethernet cable to my flat upstairs
3 pair ethernet cable is connected with a static in-line coupler to a short 2 pair ethernet cable to my videophone handset on the wall

This setup used to work no problem, now for some reason on the handset DHCP works, it is able to discover the cameras via some internet device search protocol (ping, port probe - no idea) but when I want to see the video from the cameras (I guess ONVIF) it tries to connect for about 20 seconds with a black picture and after that says the camera is offline. While I can ping the camera if I just connect a computer to that cable.

So, this used to work!? Now, if I replace the coupler with a simple active switch - everything works perfectly, also the cameras.

I’ve tried different couplers, I’ve tried another cable inside the flat, the cable I use for the videophone from downstairs used to be my internet cable - so the cable itself works, I am pretty sure (why do I get DHCP and it works with a switch?)

So the problem is that I don’t have free access to the downstairs room and the network infrastructure behind the switch downstairs is a bit of a mystery, but I am not sure how this would help me anyway.

So, there is something, on the packet level I guess (to do with multicasting?) which works if I put a dumb switch instead of the coupler.

I am breaking my head what is affected by that switch and why the inline coupler used to work and suddenly does not?

Any ideas?

I think wiring is probably poor electrically and a tiny switch you have compensates. (Or some mdi-x thing, who cares…)

To figure out the network stuff you should either find a switch with port mirroring, … or just two network cards you can bridge in software. Either would allow you to get a long term pcap dump that you can look at your leisure later in Wireshark.