@TP-Link 

+1 on getting the feature added to define as specific port as WAN port.

I also experience this issue and as My ISP do all sorts of unannounced maintenance I can loose connectivity at any time.

A few months ago I created a feature suggestion related to this but it seem to have been deleted and now I only see the reference in my subscribed topic page.

255970: Make the Deco M9 plus survive loss of internet without resulting in complete local network outage

I have somewhat reduced the impact by placing a unmanage switch (#1) in between my main DECO and the physically connected NAS and PCs but this is just a dirty work around o not loose all connectivity. This will of course not help if one of the devices restart as the DHCP server is unavailable as well during the outage.

The next switch (#2) is used as a repeater basically as I've been forced to use TP cables to the most important devices due to this issue as well.

Below is the topology. Please note that a pair of mobile phones also used over WiFi from time to time which are not represented here.

To me the issue is already clearly defined but do let me know if there is any additional information needed to make this happen.

Indeed, this added to the firmware as an option, even a non-default one, would be great.

As of now I had to resort to

1. put an old DD-WRT router in between the Cable Modem and the Main Deco
2. change the whole Deco Operation Mode into Access Point
3. re-do all IP, hostname, lease, port forwards assignments on the old DD-WRT router

(not great but now if I loose internet connection, I can keep using my PLEX server / security cameras/ etc)

Mind you, this completely removes all other benefits from the TP-Link Mesh routers since now all needs to be managed from the old router. No monthly reports or devices grouped by users, just as an example.

@djismGAMING

I completely understand why you did that and sorry to say it kind of defeat big parts of what motivate buying these products in the first place which is quite sad as you already mentioned.

If a solution isn't implemented soon I will be forced to revert to a similar setup as well as most PCs in my network rely on access to the NAS and my solution is not nearly as robust as I would like it to be.

I really hope tp-link devs come though and fix this.

@MartinJo Yeah, I got tired of waiting, since this is not a new issue.

I got a new Archer AX21 v1.20 on Amazon. Now this is replacing the DD-WRT router (flashed Netgear AC1450) I used to use since that one was giving me errors.

(though this new Archer AX21 does have it's own issues with 2.4GHz, 5GHz works beautifully)

This new router is now my main/gateway so when internet goes away, the whole LAN and wireless clients can keep working on the DECO network without problem.

Not a solution to this issue (I personally don't like to always throw more hardware at an issue) but so far it deals with it in my home.

@yacc do you have the firmware file? how is it working for you now? Did you contact them by mail or phone? I'm not in the US.. so phone is not an option. 

@TP-Link Sure. I'll send the details by mail. Thanks. 

@djismGAMING 

Sound like a good trade-off for now.

@MartinJo Indeed, I just disabled the Archer 2.4Ghz band and use the 5GHz one for nearby devices.

The Deco S4's have both 2.4 and 5Ghz bands ON to connect all the other devices.

So far it's working.