ASUS - ZenTalk

Hi,
I have a TP-Link VR600 gateway (modem/router) that I have been using for a while but the wifi range was poor. I purchased the ASUS CT8 AC3000 wifi mesh and configured as per the instruction: ISP LAN to CT8 WAN, set up as a Wifi router + node. I switched off the wifi on the gateway. All was set up fine, but started seeing warnings about using private WAN addresses and double NAT.
In this configuration I can't use the CT8 to wifi connect to my Sky Q TV system, and overall I think it would be best to remove the double NAT.
However, I am not very IT literate! I can set an option to bridge mode the VR600 and set up the CT8 with the ISP details but this has not worked so far - probably due to me! Question: Do I need to input the VLAN ID on the gateway in order for the downstream set up to work?
My main question is whether I can set up the CT8 differently to get over the double NAT issue but maintain the functions of the CT8? I could use access point but does this degrade the extra security functions within the CT8 system?
I have seen posts talking about using static IP, switching NAT off in the mesh and using DMZ. However I do not know how to do this so would welcome any instructions if anyone has any (that I could follow:))?

Any help appreciated.

Pete