I have just had FTTP from BT installed. In order to keep my Deco X73 as my main router I had to change the fillowing

1 - In 'More' under Advanced, at the top go to Operation Type and choose 'WiFi Router'

2 - Once saved, go to 'Internet Connection' in the More menu and change Type to PPPoE

3 - In username enter the username bthomehub@btbroadband.com 

4 - In password enter BT

5 - Save and you should have full fibre internet