@Mikestjames 

Have you considered switching Deco mesh to AP mode? One of the possibilites is your Main Deco overloaded when running in Router mode. See Switching a Deco to Access Point (AP) Mode.

If you are going to switch Deco mesh to AP mode, after you've made that change you should connect your 8-port switch directly to XFinity router, not through Main Deco. 

Another possibility is your Deco router runs out of IP addresses to allocate to devices. Not sure why would that firstly impact Ethernet connected devices, but this is theory worth checking.

To test that theory, assign static IP addresses to your Ethernet connected devices and see if they still lose Internet. See How to configure Address Reservation with Deco?

  @Alexandre. Thank you! I am trying the static IP address assignment for our laptops as a first step, and hope that works. On the other suggestion, the Deco 9 is the only router. The Xfinity device is just an internet modem. It doesn't have any router functions that I know of. So I can't see how plugging the switch directly into the modem would allow any IP addressing for connected devices. Am I missing something? Once again, thanks for your help!

  @Mikestjames 

You are correct, a router is required to run Deco mesh in Access Point mode. Xfinity offers gateway (modem+router combo) for its Internet users, and it also allows to bring third party hardware.

If static IP addresses do not help, it may be cheaper to get Xfinity compatible gateway and run Deco in AP mode than to replace your Deco WiFi mesh. Just an option to consider.

I have Deco M9 Plus at my house, they run fine and I am happy with their performance, but I run them in AP mode connected to my ISP cable modem/router combo. 

  @Alexandre. 

Thanks for taking the time to help me figure this out. It's very generous of you, and I do appreciate it. I'll try to pay it forward. I have always used my own modem with Xfinity to avoid the monthly charges, and I've always tried to use the best modem. I currently have the Arris S33. I would like to try your idea of setting up the Deco 9 as an AP. Do you have a suggestion as far as the best current router to put between my S33 modem and the Deco 9? I still have the Netgear Nighthawk X6 AC3000 (model R7900) that I replaced with the Decos. Would that work as the Gateway/Router? It has four Ethernet ports, so I'm assuming one would be for the main Deco puck leaving three for me to use with the Ethernet devices and/or a switch. I hate to draw on your time with my problem, but I've been retired for 5 years, and a lot of what I used to know about IT has become obsolete or has fallen out of my brain. And I was NEVER properly informed about network issues. Your help is invaluable.

  @Mikestjames 

I checked its specs, and can tell that Netgear Nighthawk R7900 is quite nice router. You should use it, to offload Main Deco. 

These are the steps:

1. Power off Main Deco, disconnect it from Arris S33 modem and from 8-port switch.

2. Connect Netgear Nighthawk R7900 router to Arris S33 modem and power router on.

3. Connect 8-port switch to one of router Ethernet ports. Your wired devices should now have Internet. Verify you have Internet connection through the router.

4. Turn off WiFi on R7900. On that model of router, you can do it with the button on router, that is nice feature. No need to log in to router configuration web pages.

5. Connect Main Deco to another router Ethernet port and power Deco on. Wait till its LED goes green. When it does, LED on all Satellite Decos will go green, too.

6. Switch Deco mesh to Access Point mode: https://www.tp-link.com/us/support/faq/1842/

7. Wait till LED on Main and Satellite Deco goes green again. All wireless devices should seamlessly reconnect to Deco mesh, but those that won't - you may need to reboot them or power off/on.

After that, you'll have your home network managed by Netgear Nighthawk R7900, wired traffic bypassing Main Deco. That should improve peformance of Deco mesh substantially.

  @Alexandre. 

I now have the Nighthawk R7900 as my router. I've lost some bandwidth because it's a slower (older) router than the Decos. I was getting about 900mbps through the Ethernet port on the Deco, and I only manage 300mbps with the R7900. But overall, I think this is a better solution because it should spread the load.

However (there's always a but!): The problem persisted with the two laptops, both current models, a high powered gaming MSI and a very nicely equipped LG Gram. The good news is I've narrowed it a bit. It seems to be associated with Steam games. Whenever playing a Steam-based game, even one that doesn't require connectivity, it will play five minutes, than I can see the Ethernet connection get shut down. I HAVE found a way to avoid it...enabling the McAfee VPN seems to work to keep the connection active, but it further throttles my speed down to about 200mbps. I'm thinking it was NEVER a hardware issue, but something being changed by either a Windows11 update or a McAfee update that sees Steam as a dangerous connection. I'll keep fiddling with connection settings to see if I can fix it, but at least you know it was never TP-Link hardware that was causing it.

Thanks again for your help. I learned some things, and i've probably improved the reliability of the network overall by using the Decos as APs instead of carrying the load for everything. I am still puzzled why the shutdown only affect Ethernet adapters and not WiFi adapters. ??

  @Mikestjames 

Your current setup, with Netgear router, should work better for you. I am only concerned with 300Mbps speed - Netgear router should be able to handle 800-900Mbps through wired connection with no issues.

I am assuming you are getting 300Mbps by running speed test on wired PC, not "Test Internet Speed" in Deco app. Also, I would suggest, if you haven't done it yet, install OOkla speed test app on PC/laptop and run speed tests with it, instead of www.speedtest.net in a browser.

It is possible that Dynamic QoS is enabled in your router. See page 42 of the router User Manual: https://www.downloads.netgear.com/files/GDC/R7900/R7900_UM_EN.pdf 

If it is enabled, disable it. Also, check for other traffic management features of your router and disable them. 

  @Alexandre. I actually went back to the Deco 9+ as router. In doing some research reading on the R7900, it's a problem many complain of and probably explains why I replaced it with the Decos. Ethernet speeds nowhere near the rating for the Ethernet on the R7900 widely reported. I'm obsessing about an Ethernet problem that appears to have NOT been a Deco hardware problem anyway, and I don't know why; if we end up running the laptops on WiFi along with everything else, the speeds are still much higher through the Decos than Ethernet through the R7900. :)

Thanks for your kindness and help! I gave you great feedback on the forum.