@TP-Link 
Still, almost a year now, waiting for a fix for my Deco M5 trio with HW version v3.0.

I am tired of resetting and re-configure my M5 and powring down my router to workaround your bug few time a week

  @TeePeeLinkSucks yeah it's pretty bad it has to be this way.  I managed to get it working again by following the plug back in one at a time.  I'm going to keep fighting with TP Link support to ger custom configuration files specifying my WAN and LAN ports so I don't end up in a routing loop that shuts off my WAN when the ports are all hardwired.  I like the speeds I am getting and the coverage across the house.  It works for my family.

However, I am definitely considering setting up my old network which is an ASUS Gamer Router which has a much better user interface and all.  I'm not sure if these devices can be configured to bridge off of my main subnet so I would probably create a route to it instead and secondary /24.  

I did get it to work though by unplugging, doing one line at a time starting with WAN.  For my 2 switches, I plugged one device in at a time.

  @TP-Link Welp, glad I found this post. I'm returning my X55 system because this exact issue happened today after my internet blipped. I Couldn't get it to connect, tried troubleshooting for awhile and finally I unplugged the cable from the ONT into a different port and it worked. This was after multiple restarts of the system. Ridiculous. Going with ASUS XD5 since I've always had good luck with them, not to mention the customization and FREE features, unlike TP Link. I don't think this system has been updated with the so called "Fix" as stated on page 350,000 of this thread.

  @ETANET 

Your description is a logical way to recover a system - the issue is that it should not fail with the internet is removed - and so you should not be put to this inconvenience!

and it doesn't matter if its a router or an access point - the Deco products all misbehave in the same way when their access to a cloud service is removed. That is completely unaccaptable!

and mine had the recent firmware update self apply yesterday - at which point they wiped the 'guest' network settings and changed them to someting I have never used! this is also garbage behaviour.

unfortunately I've had my devices too long before finding these issues out - so I cannot return them, other wise I would!

  @bcamp4390 

Thank you so much for your solution - it helped me get my LAN back (weird statement...)!

Can confirm my implementation of @bcamp4390 worked for me:

1. Make sure that in the DECO app >>> More >>> Advanced >>> Operation Mode is set to “Wi-Fi Router” and your WAN-router is in bridge mode.
2. Shut all DECOs down = unplug all power cables, keep the LAN/Ethernet cables connected! If PoE: Unplug related Ethernet cable. The WAN-Router can stay active/online.
3. Power on/plug in the main DECO unit with its “WAN in” Ethernet cable (from your router in bridge mode) attached and wait until the green LED is steady and shows an Internet/WAN connection in the DECO app. While starting up, it may be confusing (at least for me), that DECO units tend to start up (the steady yellow/light greenish LED) and switch directly to the red blinking LED (= no connection) but turning into a steady green LED (= connected) shortly afterward, so be a bit patient. At this moment, the auto-sensing should've set the WAN/LAN choice on the main DECO to WAN and everything else to LAN. DO NOT SWITCH OR MOVE THIS SINGLE ATTACHED ETHERNET CABLE.
4. Power on/plug in the first network element with an Ethernet cable connected to the main DECO unit. Network element means: If you have a network switch or LAN endpoints/computers that directly connect via Ethernet cable to the main DECO unit, it would be best to plug this in first, before any DECO satellite units - At least, that was my experience. Seems like normal endpoints don't fool around that much, as they don't have the extensive router/mesh/blabla stuff going on. They just want their IP via DHCP and get an internet connection from the main DECO unit, and can't fool around with WAN routing capabilities. If your first LAN attached device is a DECO satelite, power it on/plug it in with its Ethernet cable attached and wait for the green LED (see 3 above). PoE devices would be the last ones.
5. Repeat step 4 until every LAN device is connected, then power on/plug in Powerline/PLC connected devices as some (PX50) directly make use Wi-Fi repeating/extending and have extensive routing and WAN/LAN auto-sensing capabilities.
6. Power on/Plug in the rest of DECO Wi-Fi only satellites slow and steady, always wait for the steady green LED before turning on the next one.

Additional Note: I activated "WAN Unicasting" when accessing the main LAN IP set in the DECO app (for example: 192.168.1.1) via browser and navigating to Advanced >>> System >>> System Parameters 

@TP-Link 

I think this underlying problem is also the cause for multiple threads throughout different forums on the internet where people ask how to set a different signal source! They wonder why that DECO satellite/router connects to the wrong DECO satellite/router because the LAN connection is missing and what's left are the Wi-Fi and powerline signals! That was also my case: I stumbled across this thread only with little hope and before searching and reading a lot about (no proper available) signal source selection, as I was just seeing that my DECO satellites did connect via weak powerline or wi-fi signals far away, although they were connected directly via ethernet cable to the main DECO unit.