How to work with variable in namespace

I think I hvae a fundamental misunderstanding of namespace and/or static variable. But I have tried this test code (typed by hand, forgive typos)

test.h:

namespace test{
   static int testNum=5;
   void setNum(int value);
}

main.cpp:

#include <test.h>

int test::setNum(int value){
   testNum=value;
}

int main(){
    test::setNum(9);
    cout<<test::testNum;
}

when I run this I get the value 5, not 9 as I would have expected. It seems almost as if I have two instances of the testNum variable, but that seems to be the exact opposite of what static should be doing. I'm guessing I've made a mistake in assuming that these features were identical to their java equvilants somehow...

I also get an error stating that testNum is declared multuple times if I remove the static from my declaration of testNum, could someone explain why that is the case as well?

Thank you