@orlasttoy,

I do not really see why your attached setup would cause an issue.

The lack of support on that switch basically intercepts the packets that are needed for the Ethernet Backhaul, along with some other types of multicast packets. As long as the only odd behavior you are seeing is in regards to the backhaul, you will be fine, as the backhaul traffic just needs to not pass through the switch.

What is odd is the fact that all your wired connections are slowed, but your wireless is not. I would expect all wireless traffic to be also affected if they were using their wired backhaul.  

  @orlasttoy,

I think you found the main cause of your problem, the switch - but where you are moving it to, which may cause problems with the setup.

Looking at the proposed configuration from above, the switch traffic will not pass through your main Decos at all, meaning your devices cannot be assigned an IP from your Deco network. You would still need that switch to be downstream of the other Decos. If it is 'outside' of your Deco network, then the Deco cannot assign IPs to the devices, and instead the switch and your main Deco will fight for the one address that can be given out by the main modem. 

Also, since the graphic mentions Modem/Router: does the device from your ISP only act as a modem for your network or is there also wireless functionality enabled on it?

If it has wireless functionality, then you would need to enable bridge/IP Passthrough mode, else you would need to configure the Decos to work in AP mode and pass the network management off to your ISP Gateway. Having two 'routers' operating on the same network could cause a slowdown in performance across your entire network.

Recommended:

ISP Gateway with Wireless Functionality Enabled:

- Place Decos in AP Mode and Your Proposed Setup will Work:

Main Deco > Satellite Deco > Wireless Decos 

Internet > Gateway w/ Wireless   >

Switch

- All Devices including devices on switch will receive IP addresses from ISP Gateway DHCP Server

ISP Gateway in Bridge or Passthrough, or a Default ISP Modem:

- Internet > ISP Gateway/Modem > Main Deco > Satellite Deco > Switch for Client Devices

- All Devices will Receive IP Addresses from the Main Deco DHCP Server

- I believe that device traffic will pass normally through a switch to the Deco network, just avoid using it for mesh ethernet backhauls.