I think I've worked it out but as always I feel safer checking with the community here.

I have 3x Deco X20s through the house connected in AP mode to my ISP supplied router. I did experiment with WiFi router mode but the ISP router didn't like it. However I've been told that I will soon get fibre to my house via Openreach (UK). I understand I can plug my main X20 direct into the ONT. I assume changing to router mode ?

That leaves me with no LAN so do I use the second port on the main X20 into a switch and then onto my LAN setup ?