Create a "General" topic in the forums (and why isn't my topography working)?

Oh, and allow legacy products under Associated products. I have a network topography problem that's taken me some time to nail, and describe. 

I got me a used Netgear M4100-26G. Not bad. God price. Works well. But I can't select it as an Associated product because it's not produced any more. Like the community can't discuss legacy products?

I am also using a NetGear EV6200 as a WAP which is central to this topography problem (which I also can’t select as an associated product). In a nutshell I have a long term functioning topography as follows:

Working for years: D-Link switch --- Netgear GS108 --- Netgear EX6200.

The Switch is where all my LAND devices come together. The EX6200 works as a WAP and has IoT devices reporting on it. Has been fine for years. The GS108 is only there for historic reasons because a few other devices connect to it (a TV and an Xbox to be precise). These could actually be routed by the EX6200 and I could get rid of the GS108 but for physical topography and cable lay issues for now, but that is a digression. Point is that's a legacy working system I can manage the EX6200 and reach its web interface from any device connected to my LAN (via the D-Link switch) and the devices connected to the EX6200. Wi-Fi connected devices have internet access through the switch.

The D-link started to play up (basically making a load of oise, likely a fan issue) so I looked quickly online for ana affordable replacement with around the 24 ports. Found a chap Netgear M4100-26G. Pretty nice working well, except for one link and only one (of the 20 odd on it).

I swapped it in place of the D-Link switch and tested all my links. This one mentioned here fails. It is now:

Not working: Netgear M4100-26G --- Netgear GS108 --- Netgear EX6200.

I should point out that some 20 odd other services on the M41 do work. that this link to the EX62000 does not work on any of the M4100s ports, it's not port specific. So I tried an experiment. I still have a couple of Netgear GS108Ev3s on hand which predate the D-Link and were used while I could get by on them (but I needed more ports at some point so replaced them with a cheap D-Link switch). But they were still there and available as warm spares. Switched one on and tried this:

Not working: Netgear M4100-26G --- Netgear GS108Ev3 --- Netgear GS108 --- Netgear EX6200.

And now I can reach the EX6200 and my IoT devices are online. I can switch  between these two topographies and have done serval times now because it beggars believe, and run a ping to the EX6200. The cable between the M4100 and GS108 is some 15m through walls/ The other cables are all short and under 1m. If I plug the GS108 into the M4100 the ping stops, if I plug it into the GS108Ev3 it starts I can go back and forth, with patience (as the timeout is a but long, but have established beyond any doubt that one topography works, the other not.

So the D-Link worked with it. The M4100 for some reason doesn't like this 15m cable to the GS108. Is it the 15m that's to blame? Categorically not, at least 3  other lines emerging from the D-link (now the M4100) are 15 m cat6 cables that run to WAPs across the property The M4100 is happy with all those. These are all stock 15m cat 6 cables manufactured thusly. Three of them have the remote end manually patched into a wall plate.

There is nothing to implicate the cable here. The M4100 is implicated. I have a workaround.  But do I need to run a whole other 8 port switch just for this to work? What can I do to learn more from this, to turn into an experience gathering exercise, a learning exercise. What in the technology employed here could be causing this, and what does the M4100 offer as tools to diagnose?

It does have a cable test feature (the M4100) But it's odd in the extreme as it reports failed cables that are working (including the uplink cable - to the WAN router, which is very sort running to my gateway router) and reports cable lengths that are totally wrong by an order or two of magnitude for the ones that pass. The M4100 does have later firmware available and I may try and update some time, but it's never risk free, and the web interface upload isn't working (these old boxes have among their lovely features sometimes reliance on deprecated web features, so I'll have to look a console approach).

I test connectivity from a desktop using ping and trying to load the EX6200 web interface. And I have drawn the two topographies for the visually inclined and attach them.