[Speed] Not getting new promised speeds. Router issue?

Hi. I signed up for a new HSI package. I was getting 25 mbps and should now be getting 50 mbps. However, I am still sitting around 20-25 mbps. When my modem is plugged directly into the PC, I get the promised speed. When I use my router (all wired, no wireless), I get the throttled speed. Is there a setting I need to change in my router?? Modem: Motorola Surfboard SB6121 Router: Linksys WRT54G