Your Sly q box and or mini boses if tou have them act as part of a MESH wifi netwotk and helps to fill help fill any blackspots in your home. So your Q box in essence is like a mini router and is why your able to take a wired connection from it. However I would have thought this would have needed enabling in the settings.
Ive gone back to VM mainly for the internet now but, I did find the Sky Q and mini boxes took all the wifi bandwidth even though they had cat5e going to them. So this maybe what you were experiencing.
Like @Joss I have ethernet cable running all over the house and for my CCTV, with a mixture of cat 5e and 6. I also have a Netgear gigabit switch behind the TV cabinet.