@Sus_Deco 

If you need WiFi mesh that stays up during Internet outage, go with different brand.

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You need Internet to install Deco mesh. You need Internet to manage Deco mesh with Deco app. There is no Web management UI for Deco mesh.

Looking at this and other design decisions, it becomes clear that keeping Deco mesh up without Internet was not considered a necessity. Deco mesh original concept, which is reflected in their M-series (such as M4, M5, M9 Plus), is to have simple to use WiFi mesh for typical household without much knowledge of networking.

Deco mesh design concept is "it should just work." There are no pages after pagers of configuration parameters you could tweak. There is not even ability to set WiFi channel.

Each Ethernet port is WAN/LAN autosensing, while on many routers it is only port 1 and the rest are LAN.

I can keep going, but you should get an idea by now.

When Internet in my "typical household" goes down, and I run Deco mesh with M5s and M9s, my household members don't ask why they can't print on networking printer or why scheduled Windows PC backup to NAS failed.

They will be concerned with TV not working (it is TV over IP), landline phone down (it is VoIP), YouTube videos not playing. The youngest member of my household will be asking why she can't watch cartoons on her tablet.

When Internet is down, I can power off all Deco and rest of networking gear at my house and nobody will notice the difference.

With time and new Deco models, original Deco mesh design concepts become diluted: you have more settings to configure, Deco mesh becomes more tunable (which is not always a good thing).

Still, considering its underlying original code and probably some TP-Link Deco senior software developers still sticking to original design concepts where they can, if you need WiFi mesh where you are in charge and which can run as an insulated from Internet island, consider different WiFi mesh brand.

As for me, while I've chosen Deco brand of WiFi mesh almost by coin toss, it is proven to be a good choice.

The primary criteria of WiFi mesh success for my household is I no longer hear complaints about WiFi issues. Had plenty before I bought Deco.

I bought my Deco mesh in October 2020. I plan to keep my M5s/M9s way longer than I anticipated, until either their hardware fails and I can't buy M5/M9 as a replacement, or I get device in my household which needs at least WiFi6 and can't run on WiFi5 and 2.4GHz. 

  @Alexandre. I guess you're running a home system and your users are only worried about their streaming and social media. They can get that using their phones and mobile data when the Internet is down.

I run a small business from Home and when the Internet is down I can't print or access my NAS.

So if you don't care about having a reliable LAN, go ahead and buy a Deco.  If you want a reliable LAN buy another brand.