@Xenomes This network diagram does not fit my theory of what might go wrong, so I am going to provide other troubleshooting recommendations and perhaps one of them will help: 1. Replace Ethernet...

@koswo An example of powerline adapters troubleshooting resource: https://www.amazon.ca/vdp/04e47249121748a081dbb46fb5a2bc2d

@koswo Get pair of standard powerline adapters like these: https://www.tp-link.com/us/home-networking/powerline/tl-pa7017-kit/ After you configure them and verified they provide access to your home...

@WiCreekPaddler You should enable Fast Roaming back, it is responsible for telling mobile device to move between WiFi mesh nodes. This is unlikely to fix the issue you have, but disabling it degrades...

@trogvar Start with the following TP-Link document, which discusses Ethernet backhaul: General questions about Ethernet Backhaul feature on your Deco

@koswo Deco P9 is capable of using powerline, but when it can't - troubleshooting becomes a challenge. There is no user exposure to inner workings of powerline connection of P9. You said you added...