Welcome to the forum.
As you are posting this on the broadband forum can you confirm that you are on standar ADSL broadband, not fibre. Assuming this is the case you have probably been given. plusnet Hub Zero. The wireless performance of these is best described as pathetic unless you live in a small flat. My first recommendation would be to consider going onto Ebay and buying a BT Smarthub 6 for very little money and use that in place of the Hub Zero. You will find the instructions on how to set it up in this forum.
Do I assume that the extender you are talking about is a powerline pair? These have two units, one at the router end connected by an Ethernet cable and the second in the room over the garage? If this is so and it's OK when connected to your Skybox via an Ethernet cable then first of all I assume that the extender is one with wireless and the wireless is switched on? Secondly are you making sure that you are connecting to this and not still trying to connect to your router? Many of these devices work on a different SSID and possibly different password. When you are in the room over the garage look at the available networks and see if you can see the stronger signal than from your router and if so connect to it. Sorry if this is obvious to you.
I have no idea what industrial level kit is, it sounds like bull's excrement to me!
Moderator and Customer
If this helped - select the Thumb
If it fixed it, help others - select 'This Fixed My Problem'