First off, does Congress have the power to tell local police departments what to do? Kind of. Its complicated. The U.S. Constitution doesnt give the federal government power over local or state police. And the 10th Amendment says any authority not specifically granted in the Constitution is left to the states, so Congress has little direct authority over state and local police. But what they do have is control over money the federal government sends the states, and grants from the Justice Department are often a significant chunk of local police budgets. So they cant tell police what to do, but Congress can withhold money unless police departments act in a certain way.
The federal government has no authority over local cops, other than extortion under threat of withdrawing the federal teat.