@Splatterly 

Yes, switching off router WiFi is recommended configuration after you have Deco mesh up and running.

  @Alexandre. 

Thank you. Can I ask why this needs to be done?

  @Splatterly 

Deco mesh runs its own WiFi network, it won't share or extend router WiFi even if they configured with same SSID. Think of Deco mesh as a WiFi router, the one that can cover large distance with WiFi signal. 

Now, the question becomes: "why it is not a good idea to have two WiFi routers broadcasting WiFi signal, side by side?" One of the reasons: you only need one (Deco), another one creates additional WiFi interference you really do not need. Another reason: suppose, you run both routers with same SSID. Troubleshooting this becomes a headache as you wouldn't know (unless you dig in) which of these two routers your device is connected to.

  @Alexandre. 

Ah, now that makes sense. Would that interference also be the reason why my Deco drops Internet?

To be honest, thats what's stopping me from turning off the WiFi on my router, because the Deco dropped the internet

  @Splatterly 

If Internet dropped completely for every device connected to Deco mesh at once, the reason may be somewhere else, but disabling WiFi on router is still necessary step in troubleshooting.

If Internet drops briefly on one or few devices, this is another reason why disabling WiFi on router is necessary: device may decide to switch to stronger WiFi signal, but because there is no true WiFi roaming between Deco and router, device will drop Internet connection when switching between them. Inside Deco mesh, roaming between Deco units should be seamless and with no WiFi interruption.