@ChrisNewbold 

To summarize what you have: you have ISP router (SKY) that you can't switch WiFi off, and you have Deco mesh connected to it that you run in Access Point mode. 

With that, you do not have to switch SKY router WiFi off. Just follow simple recommendations I'll list below, they are for running multiple WiFi networks with little or no issues.

Recommendations:

1. Keep distance between Main Deco and SKY router, at least 1 meter between them

The reason would be to avoid interference between two devices transmitting WiFi signals, which will happen if you put them too close to each other. Having SKY router and Main Deco too close will result in degradation of internet speed for wireless devices connected to SKY and to Main Deco. That includes Satellite Decos wirelessly connected to Main Deco - they won't perform well if WiFi signal around Main Deco has a lot of interference.

2. Give SKY WiFi different SSID (network name) than you use in Deco

By doing that you will be in control which wireless device connects where: to Deco mesh or to SKY router. Probably, you'll prefer most of your devices to be connected to Deco.

3. Tune WiFi networks configuration on your wireless devices

On each wireless device you want to connect to Deco mesh, check what WiFi networks it knows and make it forget SSID of SKY router.

On each wireless device you want to connect to SKY router, check what WiFi networks it knows and make it forget SSID of Deco mesh.

This is just little housekeeping and will save you some time later if you'll need to figure what's going on with your WiFi networks, why wireless device has slow Internet, or why does it not show in Deco app while definitely connected to the internet.

That's it!

My son has a playstation and I think there might be an issue for him in terms of double NAT.

As long as you run Deco mesh in Access Point mode, there will be no double NAT issue. It only present for Deco mesh that runs in router mode.

Yet, due to specifics of mesh technology, you (he) might find it necessary to keep playstation connected to SKY WiFi. As long as it gets good WiFi signal from SKY router, it may do better than when it is connected to Deco mesh.

Same housekeeping rule applies: in playstation WiFi configuration settings make it forget (delete) SSID of Deco mesh and make it connect to SKY SSID.