@krisw 

So, you changed router and updated Deco firmware. Two changes, always challenging to figure which is responsible (or both, in combination).

As a troubleshooting tip, I suggest to power off Main Deco, unplug and plug back both Ethernet cables connected to it, and each from both ends. Also, plug cables to different ISP router port and different Ethernet switch port.

You were unplugging cables when switching routers, perhaps one was not plugged back tightly.

If that does not fix the issue, here is workaround. In Access Point mode you do not need to have all traffic pass through Main Deco. Connect switch directly to ISP router and Main Deco to either ISP router or to switch.

By doing that, you'll bypass Main Deco for Ethernet traffic. If problem is with M4 firmware, it won't manifest itself now. If problem remains it means it is somewhere else, Deco is now excluded from the list of suspects. 

  @Alexandre. 

Those are two changes that have happened recently, however both were not at the same time.

Timeframe:

24th Oct - changed ISP and to a full fibre package, so a new modem/router was supplied. It was an almost literal swap in/out type thing with regards to ethernet connections. Everything that was plugged into my old router, was plugged back in to this new one, with once exception as there was one less available ethernet port. The ethernet cable connecting the M4 to the router remained the same. Everything worked fine.

30th Oct OR 1 Nov - I updated the M4 firmware. I only think it is one of these dates as looking at the Oct/Nov report on the app, those days apparently had zero connected network devices, so I'm guessing something with upgrading the firmware messed with that stat. Other than that I can't recall when I did it, but around then feels about right I think.

5th Nov. - first time I become aware of the issue. Posted about it here - https://community.tp-link.com/en/home/forum/topic/175174?replyId=1147748, after spending ages trying to figure out what was going on, I restarted the Deco and it worked fine after that. Prior to restarting, I tested another device connected directly to the router/modem via ethernet and it operated at expected speeds, and this combined with wifi operating at full speed thereby confirmed it was only traffic going through the ethernet ports of the M4 that was affected. 

8th Nov. - problem has manifested again. 

I'll try that powering off, unplug and replug and switching ports.

Unfortunately I can't have the switch connect directly to the modem/router, as there are not enough ports on the router, unless I get another switch to connect to the router at the router and connect my switch to that switch... Think I'd prefer to not go that route!! 

  @krisw 

ISP routers usually come with more than one Ethernet port. If yours has at least two, it means you can connect switch directly to the router and whatever connected to another port - connect it through Main Deco.

There are no settings in Deco for traffic throttling in Access Point mode. Which means, you will need to accept workarounds. 

These things might have helped with troubleshooting:

1. Deco M4(V4) has beta firmware that should alert if Ethernet link drops to 100Mbps. Check your Deco M4 firmware version, if it is V4 you can try it: https://community.tp-link.com/en/home/forum/topic/581554

2. Some ISP routers report negotiated link speed for their Ethernet ports. Check if yours has that feature.

3. Some unmanaged networking switches have LEDs that report not just link up/down but also link speed. Next time you will be replacing your switch, consider that type of switch. Example:

  @Alexandre. 

Yeah my router has more than 1 ethernet port. I can use one for testing, but I thought you were meaning for permanent use, in which case all the ports are being used by other devices more local to the router than where the switch is. 

I have the V2 M4R Deco, so that alert thing won't be applicable to mine unfortunately. 

My router has an option to select ethernet type, fast or gigabit, and I have gigabit selected. 

My PC that is connected to the switch indicates a gigabit connection to the switch. 

I'll do the turn off unplug and replug in thing shortly. I expect that to get it back to how it should, hopefully this time it won't be temporary though. If it is, I'll have to see about reorganising all my stuff so I can plug the switch directly to the router (the switch is in a different room with an ethernet cable going through a wall to get to the router/Deco, so will need some thinking about how to rearrange!). 

  @krisw 

In regard to rearranging, you may be overthinking it. It should be easy. Get "RJ45 network coupler" which looks like this:

Unplug Main Deco from both Ethernet cables, plug them to network coupler. You have now bypassed Main Deco and linked switch directly to router using same cables. No need to rearrange anything.

Next step is to find Ethernet port for Main Deco: it can be on router, on switch, or on device such is TV/PC/etc. Unplug Ethernet cable from that port, plug it to Main Deco and connect Main Deco to that port by other Ethernet cable.

Example: I had Smart TV where I wanted to have Main Deco. Smart TV is wired to switch which in turn wired to router. I "borrowed" Ethernet cable that goes to TV and plugged it to Main Deco. Connected Main Deco to TV by another Ethernet cable and I am done. 

With Deco in AP mode any Ethernet port can do on your home network.