The secession acts of the People of the Southern States, with the arguable exception of that of Arkansas whose legislature acted for that State rather than the People, were and still are "legal" -- in fact, they are above "legal", they are supraconstitutional, beyond the reach, and beyond the scope and review, of any government or court. They were absolute and absolutely sovereign acts of the People for which the People answer to nobody who is not the Lord high invisible God of Israel.
By the way, there is a word for someone who thinks the Government is sovereign, the highest entity in society. Look that up in your Merriam-Webster's.
If that's your gold standard for valid secession then you should probably take issue with a number of other states. South Carolina, taken into rebellion solely by act of the state legislature without referendum. North Carolina, taken into rebellion by act of the state legislature despite a popular referendum against secession. Virginia, admitted to the confederacy and taking arms against the federal government before the popular referendum was held. And so on and so forth.
...in fact, they are above "legal", they are supraconstitutional, beyond the reach, and beyond the scope and review, of any government or court.
But here on planet Earth we generally refer to those as 'rebellion'.