Connecting Granny Flat with wired internet

Hi

So in the place where I live there is a main home that is hooked up with ADSL2 with Telstra and across the yard about 5-10metres is the granny flat on top of the garage.

Now the Wifi reaches the granny flat but has considerable loss in speed/stability (barely usable). That's expected seeming it's going through two brick walls. So my question is what are my options for usable internet connections in the granny flat. They must not be on the same electrical wiring as I tried a power line adapter today and it did not work unless the adapters were both either in the granny flat or the home.

There is a ethernet port in the granny flat in the kitchen however I have no idea how that could be connected to the router as where the router is connected there is only one ethernet? There are one or two other ethernet ports around the main house too but once again from what I've seen (in my previous home) there is a hub where the modem is connected that connects all the other houses to the routers LAN.

Really would appreciate some advice on this!

Thanks.