@MH52  I suppose the best way to answer your question is to give you an overview of what I've done. At least it will give you one possible way to set this up:

First of all this is what I have:

• A: Virgin (fibre) CableTV-Telephone-Broadband (200Mb).
• B: Virgin SuperHub WiFi Router
• C: ASUS WiFi Router (x4 Gigabit Network LAN Ports; 1x Gigabit WAN Port)
• D: Deco M5 (x3 units)

Before I bought the Deco this is what I had done:

1. Virgin SuperHub set to "modem mode". In other words I turned off the WiFi and Router capability so that I could use my own superior/powerful WiFi Router (ASUS). In essence this turned my SuperHub into a rather 'expensive' dumb modem which gave full 200Mb via one of its ethernet ports connected to the WAN port of my ASUS Router.
2. ASUS Router providing WiFi and fixed LAN capability throughout the house.
3. WiFi Name: "CHERRY"

But I couldn't get a signal to the back of my house/upstairs/garden. And I wasn't happy with using a Netgear WiFi Extender which meant having a secondary SSID (or WiFi name) - "CHERRY_EXT1" and so forth. (some will argue that the Extender could also be called "CHERRY" as well, which is true, but this will cause conflicts and issues).

Then I bought the Deco and this is what I have now done:

4. Followed the instructions and connected the 1st Deco to the ethernet port of the ASUS Router using the TP-LINK Deco smartphone app.

5. Connected a 2nd Deco in my son's room (wirelessly).

6. Connected a 3rd Deco in the guest room, located upstairs at the back overlooking the garden (also wirelessly).

7. The Deco WiFi is called "MANGO". The Deco is configured in "Router" Operating Mode by default.

8. THEN I did the following: I turned off the WiFi on the ASUS Router (no more "CHERRY"). So, with WiFI turned off it's now it's become a rather expensive BUT very powerful LAN Router/Firewall/Switch - which is what I expected and wanted. (I even removed the six antannae from the ASUS).

9. I then changed the Deco's Operating Mode from "Router" to "Access Point". This means the Deco is now just a plain and simple, but expensive, WiFi 'roaming' system. It no longer offers any firewall capability, port forwarding, NAT, QoS, DHCP, etc etc. Again, it's what I expected and wanted.

10. The WiFi in the entire house and garden is "MANGO". With no disconnections, no loss of service and typical speeds between 185Mb and 220Mb download and 20Mb upload.

I didn't want to decommission my ASUS Router. Why? Because it's an all powerful Router with advanced features: so it's doing all the hard work because of the 1Gb RAM and Quadcore CPU inside it; hard work such as - firewall, parental controls, NAT'ing/port forwarding, DHCP and fixed IP addresses, QoS and bandwidth limiting per device or application (e.g. NetFlix, Amazon Video), network prioritisation, Intrusion detection/Intrusion prevention, network traffic analyzing, VPN etc etc. I don't care about any of those features in the Deco; it's purpose - for me - is to offer WiFi and nothing else.

In your example you don't have an ASUS Router, therefore keep the Virgin SuperHub in Router Mode (not Modem Mode) and turn off the WiFi, thus the only WiFi is coming from your Deco. But also put your Deco into Access Point mode, otherwise you will have two DHCP 'servers' issuing IP addresses and causing conflicts. (The SuperHub will also continue performing as a Firewall etc).

The alternative is: put your Virgin SuperHub into Modem Mode and keep your Deco in Router Mode so that it does EVERYTHING. That's not a compromise: the Deco will happily perform as a firewall, QoS etc etc.

As a footnote: My ASUS continues to serve as a switch: connectivity to other switches which physically connected to my TV, Cable box, Network Storage, Bluray player, SONOS speakers etc etc.