1.Reset your router with no personal configurations and see if it fixes the issue. (search online you may find how to reset it. looks like it is a little different then other older routes.)
2.If not then it might be a hardware issue. I had the same router with the latest firmware but has no such issue.
3.if speed back to normal after reset, then try to add your configurations back one by one to check if any of your configurations cause the issue. I has a problem with my previous RT-N66U, It has a speed issue when I turn on filter and parental control.
4.also double check cable connections.