TP-Link router problem

A couple of months ago, I bought a new wireless router (a TP-Link TD-W8968). It's been working like a charm until yesterday when internet speed suddenly seemed to drop significantly to the point where it's almost impossible to open any webpages (I do get some connection but it's extremely slow). I know it's not a problem with the ISP since it works perfectly fine when I switch back to my old wireless router. It also seems to work if I connect to the TP-Link with a LAN cable. Any ideas what the issue might be? Could it simply be a faulty router and if so, is there any way to verify this before I contact the seller?

Edit: Actually, let me take some of that back, when I connect with a LAN cable it seems to work better initially but it's like connection slows down in the extreme after a minute or two. Still works fine with my old wireless router though.

 

Sorry for all the edits but I've fiddled around with it some more and I now believe that it is an ISP issue after all. When I said it was working fine with my old router it was, until I tried accessing data-heavy websites or tried downloading files in which case it's peaking at 10-15 kb/s. I've called my ISP and they're looking in to it.