"Whomever" can only be the object and never the subject of a sentence. "Whoever" can be but that has a different meaning than anyone. For example if someone asked "Who should do this?" The answer "Anyone can do it easily" implies that the thing is very easy to do. But the answer "Whoever can do it easily" implies that the thing is not easy and only a subset of people can do it. Whoever is "quienquiera".
Jan 26, 2019 - A relatively clear explanation here: https://spanish.yabla.com/lesson-Cualquier-Cualquiera-and-Cualesquiera-550
Summary: "Cualquiera (anyone) is an indefinite pronoun and cualquier is an indefinite adjective." However, I highly recommend that you carefully read the article.