@Siem87 

It is possible your ISP Router loses Internet connection, and Deco just exposed it. This is what have happened in my house after I deployed Deco mesh. I am not online 24/7, so most of Internet drops went unnoticed before, but with Deco LED light in front of me every time Internet connection went down I saw it with LED switching to red.

I would suggest to focus on ISP Router first. Some routers offer uptime report and some have System Log where you may check connectivity. My router, for example, has this in one of its management screens:

LAN is for connection to home network, WAN for Internet. This says my Internet connection is up for 10 days and 23 hours. When Deco LED went red, I'll check that router screen and can see WAN Up Time reset to few minutes. That told me there is an issue with Internet, not with Deco.

You should check with router documentation or with ISP Tech Support if your router reports Internet connection uptime.

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You can switch Deco X60 to Router mode. Most apps and services would not care and work just fine with two routers. XBOX will not like it, and if any app does not - you'll see it right away. You can always change Deco back to AP mode if you have those.

Running Deco in Router mode will not fix underlying problem, but it might mask it. For example, if you have in house file server from which you stream video to TV, in Access Point mode when Deco LED goes red that streaming could be interrupted, but in Router mode it should not be.

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Also, the issue might be a simple as bad connectivity between Main Deco and ISP Router, but I am sure you've already checked it with different Ethernet cables, using different Router and Main Deco Ethernet ports.

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Deco in AP mode may also turn red if your ISP has DNS problems, because Deco uses DNS queries for Internet health checks. This is most unlikely scenario, especially if you are using large ISP provider, but if TP-Link Support tells you to change DNS servers in your configuration settings you now know where they are coming from.

@Alexandre. 

Thanks for the time you took to help me out, i appreciate it:

1. when i use the router without deco i do not have any problems. Since i had lots of videocall, i would notice. So i do not think the router itself is not the rootcause. 

What i noticed yesterday is that in the DECO app; i saw 2 different devices with the same IP address. It was my switch and a wireless device. After that i gave Both DECO's and the switch a static IP ( I felt like MCGyver doing this and don't have a clue if it helps).
I also downloaded a network monitor app to see if this happens again. 

If the redlight comes back, i will try to use the deco as router and see if that changes anything.  But then i have to make sure, the DECO gives out other IP ranges then the KPN router right?

@Siem87 

TP-Link TL-SG105e is unmanaged switch, it should not need an IP address. That topic is discussed here: https://homenetworkgeek.com/does-a-network-switch-have-an-ip-address/

There is something else going on with your home network, or you perhaps have different type of networking switch.

Of course, if something is messing with your IP allocations, resulting in conflicting IP addresses, that might get exposed by Deco as well. Also, if you have static IP addresses configured on devices instead of router, that might create IP address conflict when using ISP router and will when you switch Deco to router mode. 

In router mode, Main Deco will provide IP addresses in 192.168.68.* IP range to your home network. That should be different from your ISP router IP range, but as long as you do not hardcode IP addresses on devices themselves, they should pick new IPs from Main Deco and work just fine.

@TP-Link 

I Believed it was the google chromecast.

@TP-Link 

unfortunatley the error still excists; multiple times per day i receive a red ligt

I connected 1 device to my router via cable, and during the redlight; it still had internet connection. 

Only thing i can try is to put it in router mode, or return it to the shop i guess