iphone 4s wifi problems

i have a new (2nd hand, new to me) iphone 4s. All set up fine but I'm having trouble with the wifi connection. I have other iphones, tablets and laptops and they all connect fine anywhere in my house and parts of the garden, I also have fibre optic so that shouldn't be an issue. But my new handset will only connect to the wifi when I'm in the same room. Drops out as soon as I leave the room. In the next room it will list my router as being available but won't connect. I've tried resetting the network but no change, the ios software is all up to date. Can anyone tell me of a quick fix?