Adding 2nd wireless router

Hi,

This summer I decided to get a garden office pod, at first I wasn't sure if the wifi would reach it, but I moved the router from the hallway to the back door and it's working well. The only problem is now the wifi is a little patchy upstairs. Right now I'm having to keep moving the router when I start/finish work... there has to be a better way of doing things. I tried a 3rd party wifi booster but it had almost no effect.

Has anyone managed to set up a second router or extend the existing wifi? I'm happy to pay for whatever gear might be needed, but don't want to waste any more money on a booster that just doesn't work.

Big thanks, Kev