If the plumbing fitting is stuck and will not break free then you should try moving the wrench in the tightening direction and see if it will tighten a bit.
Then try to work it back the opposite direction to loosen.
Sometimes a plumbing fitting may be so stuck though that you will be unable to get it apart and in that case you may have to just cut the old plumbing fitting off and replace it with a new one.
You might also need to add a metal pipe as a cheater bar to use as leverage which can sometimes help break stuck bolts and plumbing fittings loose.
You can also try hitting the wrench with a hammer or hit the side of the pipe fitting with a hammer which can sometimes break the rust free and help it come apart.