I have built a garden office and the WiFi signal from the router is to far away. I have taken a cat 6 network cable from the router to the building, but I would like a WiFi signal is there any device I could attach to the cable to give me WiFi
Yes, pretty much any old router with wifi capability can be added to give you a wifi hotspot
just connect the Ethernet cable to it, give it a different ip address to the hub and disable dchp on the router.
... of course if you want better quality wifi then go for a better router.
you could get a proper wifi access point, but they tend to be more expensive.
Thanks for the info I will have a look on eBay for a router.