@jgraaf3 

Was it always like that, meaning since first time you connected PC to Deco (or Deco to PC), or is it started recently?

While I am waiting for your reply, if this is Windows computer I suggest to reset its network adapter to default (factory) settings and see if that fixes the issue.

Steps are different depending if it is Windows 10 or Windows 11, just google:

How to reset network adapters in Windows 10 from Settings

or

How to reset network adapters in Windows 11 from Settings

depending which Windows version that PC has, and follow them. 

  @Alexandre. thanks for your response!

it wasn't always like this.. This issue was there for a while and we replaced the cables - issue was gone (more than a year ago). As of last week this issue popped up again..

resetting network adapters is worth a shot. Iĺl try that.

thx

  @Alexandre. tried it.. didn't help unfortunately

  @jgraaf3 

PC rebooted after network adapter reset, right? Because, if it did not, these weren't right steps or were incomplete steps.

Deco mesh in AP mode does little more than passing traffic. You know that from Deco app: other than blacklist, there are no other LAN configuration settings.

I can think of only few things which could cause such havoc on LAN. Conflicting MAC address or IP address, but symptoms do not match.

Problematic network adapter on PC, but you reset it. If it were properly reset, it should also eliminate conflicting MAC or IP address configured on PC.

Also, possibly, bad network driver on PC which were updated when issue went away and then recently updated again which brought old issue back. These updates could have been done automatically by Windows or network driver vendor software that came with PC.

It's a stretch, but ISP router might block Main Deco Ethernet link assuming it is trying something nefarious, when it is PC connected through it. Also, ISP Router blocking Main Deco Internet health checks but not cutting actual wired link off. 

I am not sure if you have access to ISP Router logs and if they do report anything useful. 

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One more thing I would suggest to do with PC is disable IPv6 in its network driver. IPv6 know to cause issues with gaming, which is opposite of what you have, but I would suggest disabling it anyway. Again, google for "How to disable TCP/IPv6 (IPv6) on Windows 11"

If your work laptop has Ethernet port, try it connected to Satellite Deco instead of PC and see if work laptop will break Deco mesh. It should not, but that will bring you confidence that issue is between PC and rest of LAN, not just with Deco (or if it does - could confuse me and may intrigue TP-Link Support).

Other than that, I can't give you the fix. If you would like to, after you tried what I suggested above, I could discuss workarounds with you. WiFi dongle you are waiting for is one of them, but there may be other workarounds which will let you keep wired link from PC to ISP router.

  @Alexandre. yes PC rebooted. With WiFi it seems to be ok. Looks like it has been isolated. I'll try to keep him on wfi. If that works it is good enough. Otherwise I'll continue with your other suggestions. Appreciate you help!

  @jgraaf3 

Here is another suggestion, in advance, in case WiFi connection appears unstable for gamer. As long as PC user is happy with WiFi link, no need to follow it. 

I still suspect the issue is not specifically with Deco mesh, but with specific PC network adapter and Deco mesh. Because of that, my recommendation would be to get USB 3 to Ethernet adapter. That will keep Ethernet link, but will remove PC internal networking card from communication with Deco mesh.