@SteveWex I just did in place upgrade of my M5/M9 Plus Deco mesh to set of X50-PoE. What I thought initially was incompatibility between M5/M9 and X50-PoE happened to be completely different issue:...

@Cornishman Because the problem seem to go away from the Deco mesh itself, I can only give you general recommendations. Things I could have tried myself in your situation. You have PoE switch, just...

@dkw92 Well, maybe you should try daisy chain I suggested. If that gets you Ethernet backhaul, it is not firmware or configuration setting, the focus of investigation should shift to switch and...

@OMGChris Deco Access Point mode can only work when there is ISP modem/router, not just modem. I told you that, but probably did not emphasized it strongly enough. I am glad you recovered your Deco...

@OMGChris This is correct: the next step is to switch Deco mesh to Access Point mode. You do not need to move Deco units for that or change network layout in any way, just make configuration change...

@dkw92 Check if IPTV enabled accidentally, and disable it if it is: https://www.tp-link.com/en/support/faq/2421/ Enabling IPTV is the only way to disable second Ethernet port on Main Deco I can think...