I am not a lawyer but this is not in appeal territory. This is a case where, I think, there must be a motion to dismiss with prejudice because Flynn was never told of the exculpatory evidence (Strzok's dismissal) before he had a chance to decide if he should plead guilty. Flynn was led on by Strzok who was not a disinterested party and thus, per the Supreme Court ruling in Giglio, all his testimony was invalid. Flynn's lawyer should ask for the dismissal with prejudice and then, if the judge has any sense of law and justice, he should file charges against Mueller and Strzok in court and refer them to the Bar for sanctions (disbarment).
From your keyboard to Gods ears......