For your best broadband connection, you need to swap the first socket for the one advised above and then run the router from that, running the router from the 3rd socket in line could be causing all sorts of problems.

If you want to test it then do as advised, remove the extensions from the rear of the first socket, plug the router into it and then post these stats, if there is an improvement in the stats, you know your internal wiring is the issue and you can then take the steps to fix it, if nothing changes then at least you haven't wasted you time and money.

You can easily reconnect the extension sockets once you have the stats with them disconnected, if needed take a photo of them connected so you don't forget which goes where.