Why am I getting the error 'undefined reference to `MinHeap

I am currently trying to implement a min heap, however, I am running into an issue. I am having difficulty determining why my code is not linking properly when I compile it. The files, commands, and error are as follows:

min_heap.h

#ifndef MIN_HEAP_H
#define MIN_HEAP_H

#include <vector>
#include <algorithm>
using namespace std;

template <typename T>
struct HeapNode {
  HeapNode(const T data, const int key) : data(data), key(key) {}
  bool operator<(const HeapNode<T>& rhs) {return this->key < rhs.key;}
  bool operator<=(const HeapNode<T>& rhs) {return this->key <= rhs.key;}

  T data;
  int key;
};

template <typename T>
class MinHeap {
  public:
    MinHeap() {};
    ~MinHeap() {};

    void insert(const T data, const int key);
    T extract_min();
    T peek() const;
    int size() const;

  private:
    vector<HeapNode<T>> heap;
};

#endif

min_heap.cpp

#include <iostream>
#include "min_heap.h"
using namespace std;

template <typename T>
void MinHeap<T>::insert(const T data, const int key){
    cout << "Insertion." << endl;
    if(this->heap.size() == 0){
        cout << "adding first node" << endl;
        this->heap.push_back(new HeapNode<T>(data, key));
    }else{
        cout << "not first node." << endl;
        this->heap.push_back(new HeapNode<T>(data, key));
        int currentIndex = this->heap.size() - 1;
        while(((currentIndex - 1) / 2) >= 0){
            if(this->heap[(currentIndex - 1) / 2] > this->heap[currentIndex]){
                swap(this->heap[currentIndex], this->heap[(currentIndex - 1) / 2]);
                currentIndex = ((currentIndex - 1) / 2);
            }else{
                break;
            } 
        }
    }
}


template <typename T>
T MinHeap<T>::extract_min(){                          
  T data;
    data = NULL;
   return data;
}

template<typename T>
T MinHeap<T>::peek() const{
    T data;
    if(this->heap.size() == 0){
        cout << "size: 0" << endl;
        data = NULL;
    }else{
        cout << "something in there" << endl;
        data = this->heap[0];
    }

    return data;
}

template<typename T>
int MinHeap<T>::size() const { 
    return this->heap.size();
};

customMain.cpp

#include <iostream>
#include "min_heap.h"
using namespace std;

int main(){
    MinHeap<char> heap = MinHeap<char>();

    const char data = 'a';
    const int key = 7;
    heap.insert(data, key);

    return 0;
}

I am compiling the files above with the following

g++ -c min_heap.cpp
g++ -c customMain.cpp
g++ min_heap.o customMain.o

The error:

customMain.o:customMain.cpp:(.text+0x41): undefined reference to `MinHeap<char>::insert(char, int)'
collect2.exe: error: ld returned 1 exit status

Any help would be greatly appreciated!