TP's attitude regarding this issue is unacceptable. "No WAN? No LAN" is a feature, not a bug.

Simply intolerable. Especially with the DECO units that are meant to be used with smart-home devices. All my gateways (Ikea Trådfri, Tado Radiator, Home Assistant, Philips Hue) for smart devices are hardwired with Ethernet. These all stop functioning when WAN (Internet access) is interrupted for even a few seconds. 

I cannot control my home, turn the lights off, change the temperature nor access my local NAS server. Because TP-link does not believe Local network should be functional during internet loss. 

Absolutely a major flaw with this system. Spent all day trying to set this up and having failed ethernet backhaul. Never occurred to me that the LAN port would be disabled as it auto-sensed. I had to take it off Router Mode and use my old router and use these XE75 in Access Point mode, where I lose parental controls to set limits on internet for my kids and have added device. Hey, I was considering paying the $55 a year for the extra feature! Seems this could be a firmware update if TPLink could work on it. Most important needed feature. Can't really call it a router until this gets fixed. 

  @Triple3D 

Update: I saw someone post elsewhere on a Verizon forum that they believe this doesn't work well with fiberoptic neworks where they send ethernet cable in the house from the ONT outside. I have this setup and perhaps connecting the cable directly from ONT causes this device to not allow use of the LAN ports? It was strange to me when I had the ONT ethernet cable connected to the Deco XE75 and then connecting my computer to another port gave no internet. I have Frontier fiberoptic internet. Must be continuously autosensing or something?

Bought a set of xe75's earlier in the week and was happy with how quickly they were up and running and was able to remove all my old networking equipment.

One night this week my isp must have had an outage and the switch i have connected to my main unit just became useless. There is no notification about this and so I've spent days checking cables replacing switches and running diagnostics to finally stumble upon this thread which reveals its nothing I've done but a "feature".

This needs sorting by Tplink i'm not even sure how i sort this out out now, are there any instructions on how to get everything back up and running or is it just a case of resetting and starting from scratch? If that is the case these are getting returned.

  @Bigredfury The solution will definitely be to reset and redo everything, and then you will have the same issue again as soon as your ISP goes down.

If I didn't live in a country where I can't return my Deco, I would have returned it as soon as possible, after finding out this fatal flaw. If you read all the comments in this thread then you know I'm not alone. Every customer sees this for what it is: a serious problem. TP-Link thinks they know better and that it is, in fact, a feature. It's a classic case of thinking you know a customer's needs better than they do.

I, for one, will never buy another TP-Link product.

  @Bigredfury 

As long as TP-link does not solve this "feature", there is no direct solution.

The only possibility for the network to work when there is no WAN is to put another router in front of the tp-link router and to use it as an access point.

Like:

ANY router --> Tp-Link --> Switch or any other device(s) via LAN or WiFi

My setup is something like:

  @TP-Link Can we not add an option to set which port we want to use as the wan port and which we use as the lan port?

I have seen on other systems and opnsense is like this where you can set the wan/lan port to a particular nic, but I have noticed they have the same problem that if your internet goes down, routing between local network can fail. only way i've got around it in the past is to use a linux based pc to provide dns, etc

  @TP-Link This is a major bug on your 4G failover, and you don't care about it. 

Didn't know about this when I bought. I'm sending my DECO X20-4G back for refund.

EDIT : just found this LAN stops working after unplug WAN cable from Deco X20-4G - Home Network Community (tp-link.com)

There is a beta firmware trying to fix things since april 2022 ?? why is it taking so long ?

  @David-TP 

Im internet Provider operator in Poland and we see that DecoM4 router is injecting to the our WAN Network its local traffic.

We temporary disconnect some customers unless they configure their routers according to terms and specifications of the internet stards which base thing in router is it's LAN and WAN port.

I've attached the screen of our HUAWEII GPON unit report.