Defining a class with templated array size

I am trying to define a class which has member data corresponding to a std::array whose length is variable (defined when constructed). I think my implementation is nearly correct however I am getting this error when I attempt to compile:

Undefined symbols for architecture arm64:
  "Bingo<10>::hello(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >)", referenced from:
      _main in sol-1dea7f.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

What am I doing incorrectly? So far I have:

bingo.h

#include <array>
#include <string>

using namespace std;

template<int size>
class Bingo
{
    public:
    array<array<int, size>, size> board;

    void hello(string s);
    
};

bingo.cpp

#include <string>
#include <iostream>
#include "bingo.h"

using namespace std;

template<int size>
Bingo<size>::Bingo()
{
    cout << "constructed" << endl;
}

template<int size>
void Bingo<size>::hello(string s)
{
    cout << s << endl;
}

main.cpp

#include <string>
#include <fstream>
#include <iostream>
#include <array>
#include "bingo.h"

using namespace std;

int main()
{
    ifstream input ("in.txt");
    
    string line;
    while (getline(input, line))
    {
        // cout << line << endl;
    }
    Bingo<10> b;
    b.hello("test");
    input.close();
}