So until recently it worked fine with the router, but recently dropped to 10Mbps when using the router with a wired connection; modem --> router WAN port --> LAN port to PC?
When you say it is slow, do you mean high latency, takes a long time for pages to load but then they load normally or they load very slowly -- you can test your latency at pingtest.net which will also check for packet loss.
If you connect directly to the modem with your PC and then open your command prompt box and type ipconfig /release and then next line ipconfig /renew do you get a connection to the Internet through your modem? If yes, go to www.speedtest.net and check your download and upload speeds.
What model router? What model modem?