I can't assign a value to a 2d array

So this is my first attempt to write a program using classes in c++. As an exercise, I'm trying to create a 2d-array class that can do certain things. This is a part of the code in the main.cpp that my class has to be able to run.

int main()
{
    array2d<double> ar(2,2);

    ar(1,1) = 1; 
    ar(0,0) = 2;
    ar(0,1) = 3;
    ar(1,0) = 4;

    auto X{ar};

    array2d<double> c(2,2);
    c = ar;
    c.diagonal(1) = 5; //c(1,1) = 5 
}

And this is my code. I'm getting an error in the diagonal function "error: lvalue required as left operand of assignment". I can't figure out what I should do. The purpose of this function is to assign values in the diagonal elements of the array.

template<typename T>
class array2d
{
private:
    int n,m;
    vector<T> vec;

public:
    array2d(int M, int N)
    : n{N}, m{M}
    {
        vector<T> vec1(n*m);
        vec = vec1;
    }

    array2d(array2d<T> &arr)
    : n{arr.N()} , m{arr.M()}
    {
        vec = arr.V();
    }

    array2d &operator=(array2d<T>  & arr){
        vec = arr.V();
        return *this;
    }

    T &operator()(int i,int j)
    {
        auto it = vec.begin();
        it += i*n+j;
        return *it;
    }  

    T diagonal(int a)
    {
        auto x= *this;
        return x(a,a);
    }

    int M (){return m;}
    int N (){return n;}
    vector<T> V(){return vec;}
};