They were heretics from the beginning because they did not teach what the Church taught. The is the definition of heresy. Only Ecumenical Councils can call someone a heretic. But, then, only Ecumenical Councils can change the Creed too. It was also the first Ecumenical Council that decreed there would be no kneeling on Sundays. Obviously this never stopped the Church in the west, which acted on its own, often disregarding the binding nature of Ecumenical Councils.
As far as Donatists are concerned, they were no different than Cromwell's Puritans—they represented an extreme minority. Specifically, they rejected the absolute forgiveness for repentence professed in the Gospels. Doesn't get much more heretical than that!
Donatists were what you would call extremes, ultra-con-servatives. You have similar examples among Russia's Old-Believers and various other puritanical sects all over Christendom.
Once they denied the Sacraments of the Church, they exocmmunicated themselves from the Church.
The very meaning of the word "church"(Eklesia) means those "called out of" sacrament. ceremony, tradition, and religious festival.. (John ch 10) and other places....
Even the word Sabbath is trumped by the Holy Spirit(paraclete)..
Jesus treated all seven days as the sabbath..
Example: Love the Lord your God, and treat your neighbor as yourself... 24/7..