@Bhooty 

I am guessing, your DNS server is connected to Home network. Also, I am assuming you configured Guest network to be isolated from main network, in Deco app.

If this is correct, then what you have is by design: devices connected to Guest network can't reach DNS server on Main network. Devices use DNS server to check Internet connectivity, and that's why they show "no internet."

If you disable Guest network isolation from Main, in Deco app in WiFi settings, that should fix the issue.

In case you need both networks separated, with Guest isolated from Main, see this conversation that may give you an idea how to overcome the issue: https://community.tp-link.com/en/home/forum/topic/262466

@Alexandre. 

How can i disable isolation

I only see the turn off option for guest network

I need the guest network cause my mother have a xiaomi mi max

Which keeps going on off in wifi cause it wants to use the 5ghz 😹

@Bhooty 

See screenshot below, from Wi-Fi Settings screen in Deco app. Network isolation toggle is at the bottom, named "Isolated From Main Network," and it is disabled for my home network. It seems you have an easy solution to your problem - do the same.

Note one thing: there is some sort of a bug in Deco app, when I disable Network isolation Deco app turns on 5GHz for Guest network. So, because I don't want 5GHz on my Guest network, for me it is multiple step process: disable network isolation, close Deco app, open it again, navigate to WiFi settings screen, turn off 5GHz for Guest network.

@Alexandre. 

It's not available for my m5

@Bhooty 

I forgot to mention I run my Deco mesh in Access Point mode. If you run yours in Router mode, that will explain difference in available configuration settings.