@David-TP 

Please find the lay-out attached.

I also tried to use the speeds test from out the DECO app. Same result.

  @David-TP issue remains the same, even after disabling the 2.4Ghz band. Only after a full reboot of the system (all Deco's), the speed is normal again for a while, but decreases back to 80-90Mbps after a couple of hours. Very frustrating. Maybe I should stop using this DECO system?

  @David-TP If you think that a solution can be found, I'm glad to spend a bit more time.

I did some additional tests.

I rebooted the DECO system and just after the speed went back to normal. (see attached screenshots)

However, after a while and whitout any reason the speed drops again to 80-90.

I also connected a laptop directly to the main DECO, but result is the same.

I also did some speedtests directly fromout the DECO app, and also there you can see the drop in the speed.

Regarding you last question, it doesn't matter to which unit I connect my device.

It's always OK on every DECO or bad on every DECO.

The router/modem from Telenet is their own "brand". I took a screenshot from their website with some additional info.

  @David-TP I'm sorry that I forgot to attach the screenshot of my modem, but it's the model CH8568LG.

To answer your second question: to be honest I don't know. I never payed much attention to it, even though the kids were often complaining about the speed. 

I just rebooted once and a while the DECO system and the Telenet modem, and than it was OK.

It is true that we had to change our modem a few months ago. Previously we had the CV856OE modem. After an upgrade to a Gbps internet connection, we had to change out modem as well.

It is only recently that I started to investigate the problem, because I got more and more issues with my Teams meetings. (interruptions, bad network quality etc...)

On the other hand, I may have some good news as well.

As I explained I connected the DECO directly to the router/modem Ethernet port. It is not possible to configure neither the port on the DECO or the modem. We must just assume that once connected, the speed between the both devices will be auto negotiated and  the speed will be always 1Gbps. 

Since the speeds is always falling back to 80-90mbps, I was thinking that maybe an incompatibility between the deco and router Ethernet ports might be the problem. In such a situation, it is often the case that the maximum speed of one or both ports would be put back to 100Mbps, what could explain the speed of 80-90mbps.

To avoid this "possible" incompatibility, I decided to put a small TP-105 Gbps switch in between the DECO system and the router. 

After a couple of hours testing and already 1 night later, the speed is still OK. Let's hope that this is the problem (and solution), even though I would rather see an updated OS for the deco system to overcome these kind of unnecessary problems. I'm just lucky that I have a spare switch available to do some testing on my own but customers without any spare IT materials would become crazy.