A communist country can call themselves a republic if they like, but that is a bastardization. Their elected officials are not really elected, they are appointed or run for office with no competitor. No parties exist except the one Communist party. All the republics in a communist country have the same laws unless the oligarchy gives them permission to deviate. That is not a republic.
I agree that communist countries are not republics; they’re pseudorepublics. But you did herein point out one of the characteristics of a republic, that being elected officials. Other ones are true supremacy of the people, as well as separation of powers in government, and the rule of law versus the rule of man with nobody above the law.