• cmath and iostream are not necessary in solution setup.
• a vector of fixed size 5 does not really make sense, maybe std::array<int, 5> would be better?
• Why 2 may not be considered a prime, if it is a prime number by definition?
• input collection should be passed in by a const reference.
• using namespace std; should be preferably avoided in global scope of solution snippets.
• I would suggest using assertions with custom messages which make debugging easier when tests fail (see the paragraph on assertion messages in C++ authoring guide).

Please see C++ authoring guide: https://docs.codewars.com/languages/cpp/authoring

I am going to unpublish the kata due to it not being ready for publication:

• no sample tests
• no random tests
• numerous typos and grammar issues in the description