I worked support for an ISP before.
If we didn't provide the router, then we can't support it. There are way too many variables with third-party routers for us to actually do that. In those instances, we would provide one and if it still can't deliver the bandwidth, then we will continue to troubleshoot.
That said, to rule this out, plug your computer directly into their modem or handoff. That's the best way to rule out router problems.
Side note: as someone who loved dd-wrt, I stopped using it because it was slow. Third party firmware is awesome since they add a ton of functionality but you lose a lot in performance when you do that.