Work on the sync speed reported by the router's web page first; that will be completely unaffected by MTU, RWIN etc. The sync speed is purely a function of the router and the line between you and the ISP.

Make sure all the other devices (phones, faxes, dialup modems etc) on your line are properly filtered; that sort of issue becomes more serious when the potential speed increases.

Even if you think they are, unplug everything except the router from the phone line, then power-cycle the router and see what speed it syncs at. It's rare, but those DSL filter devices do fail.

If you still get crummy speeds with nothing else on the line, it's a line quality issue, either within your house or between you and the exchange. ADSL is distance-dependent, but you shouldn't have lost that much speed just as a result of the ISP uncapping you.