I have Sky Fibre and my current setup is a BT Openreach modem and the Billion 7800DXL which has been working fine for a year or so. The 7800DXL has LAN IP's, MAC filtering and Wireless all set up nicely. Previously my setup was the BT Openrech modem and the Sky Sagemcom 2504n router which didn't meet my needs of gigabit LAN and was very restrictive on settings.
I've recently noticed the speed of my Sky Fibre has dramatically decrease for no apparent reason. I need to contact Sky to get them to have a look and run some tests, however I can't do this with the 7000DXL connected.
What I would like to do (and need some help doing) is to re-setup the BT Openreach modem and Sky Sagemcom 250n router so Sky can run some tests but have these acting as a fibre modem with all my devices connecting to my 7800DXL and obtaning IP's from the 7800DXL and using the 7800DXL's wireless connection.
Can anyone help with what settings on both routers I need to change/set?
I've tried, but failed misserably and had the household shouting there's no internet

Thanks