new wireless router has taken out my wired Internet?

I am running a bt business hub in my home, But the wireless signal wasn't strong enough to reach my bedroom. My office of which is under my bedroom had a wired connection back to the bt business hub that was working fine. So I bought a tp link tl-wr841n wireless router, which has wireless and 4 ports. So I unplugged the pc in my office, plugged it into the tp link router and then connected the desktop pc by wire to the router. The desktop then displayed a low connectivity sign. I tried to get into the tp link settings typing the ip address into the browser but it can't connect. I then had a go on my iPad and got in fine, changed the ip address as the internet wasn't working on the iPad either. This worked and the iPad wireless is working fine through the tp link. But although the cables are fine and the lights are on the back of the pc and router showing a connection, the desktop still is saying limited connectivity. What's more strange is since I plugged the tp link router in when I unplug it all and put the desktop back as it was direct into the bt business hub the computer still won't connect. Ive checked firewalls and ip automatic ip address etc. I spent ages on the phone to tp link and they don't know what it is....can anyone please help?
Thank you