I don't rely on a wifi extender in my setup using the NowTV Hub 2 Router where my wifi coverage is good throughout my home (even outside in the front or back garden).
So i won't be in a position to recommend anything from first hand experience sorry.
If you don't get any other replies on here, then if it was me i would be inclined to type in "wifi extenders" wifi boosters" or poor wifi performance in the search box on the top of the forum page to see what results come back .
I would guess there would be many return hit results with different suggestions and ways of over coming poor wifi performance from the NowTV Hub Router.
You just need to spend some time to read through the different comments or wifi extenders / wifi boosters that people have recommended or using an old Router in combination with the NowTV Hub has a wifi access point.
I also have issues with Wifi signal, anywhere outside of the room that the hub 2 is in. For example my hub is in the lounge on the back of my house and if I move into the garden around 5m from hub I get no wifi signal on my phone. I would also like to know how I can improve this as I did not have this issue with my previous providers hub.