Secession cannot be “legal.” As a revolutionary act, it is by definition outside of the constitution and the legal system. Just as the American Revolution was and could not be “legal,” as it destroyed the existing legal system and replaced it with another. That’s what revolutions do. You will note the Founders, in the Declaration, did not appeal to “law.” They were fully aware they were rejecting and attempting to replace the existing legal system with another.
A revolution is sort of like treason, which of course it is, if sometimes a fully justified treason. A revolution becomes legal when it prospers. If it doesn’t prosper, it ain’t legal.
A revolution can, in theory, be fully justified, but not by appeal to “law.” The Constitution defines itself as the supreme law of the land, and it is silent on the question. If the supreme law doesn’t specifically allow for secession, there is no higher law to which an appeal can be made.
Except, of course, for “an appeal to arms,” which is what the CSA tried, and which comprehensively failed.
The people are sovereign. There is no higher law than the will of the people. If a state votes to leave the union, then the union can only prevent that from happening by resorting to armed force against the people.