template class not finding friend function

Question

I have a template class with a friend operator, but when I try to use it I see undefined reference to `operator<<(std::ostream&, Numbers<10ul> const&)'

template <size_t Base>
class Numbers{
    int digits;
    
    public:
        Numbers(int);
        friend std::ostream& operator<<(std::ostream&, const Numbers<Base>&);
};

template <size_t Base>
Numbers<Base>::Numbers(int n){
    digits = n;
}
template <size_t Base>
std::ostream &operator<<(std::ostream &out,const Numbers<Base> &a){
    out << a.digits;
    return out;
}

int main()
{
    Numbers<10> example(1234);
    std::cout<<example;

    return 0;
}

I saw here that template classes don't work well if you declare it in the .h and you define it in the .cc, so I put it in the same file but it still doesn't find it.

I also saw a post saying to declare the functions before defining the class, as in

template <size_t Base> class Numbers;
template <size_t Base>
std::ostream &operator<<(std::ostream &out,const Numbers<Base> &a);
template <size_t Base>
class Numbers{
Rest of the file

But it also didn't work.