Linker error "unresolved external symbol" when trying to serialize an object

I have the following class:

#pragma once
#include <string>
#include <fstream>
#include <iostream>
#include <cereal\types\string.hpp>

using namespace std;

class Decryptor {
public:
    Decryptor(string clear_set, string code_set);
    Decryptor();

    template<class Archive>
    void serialize(Archive & ar);

private:
    string clear_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    string code_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
};

with the member function

template<class Archive>
void Decryptor::serialize(Archive & ar){
    ar(clear_set, code_set);
}

and want to serialize/save an instance of it.

For that, in the main function I have

{
    Decryptor user_dec;
    ofstream savefile("mono.sav");
    cereal::JSONOutputArchive archive(savefile);

    archive(user_dec);
}

When compiling (in Visual Studio) I get the error

main.obj : error LNK2019: unresolved external symbol "public: void __thiscall Decryptor::serialize(class cereal::JSONOutputArchive &)" (??$serialize@VJSONOutputArchive@cereal@@@Decryptor@@QAEXAAVJSONOutputArchive@cereal@@@Z) referenced in function "public: static void __cdecl cereal::access::member_serialize(class cereal::JSONOutputArchive &,class Decryptor &)" (??$member_serialize@VJSONOutputArchive@cereal@@VDecryptor@@@access@cereal@@SAXAAVJSONOutputArchive@1@AAVDecryptor@@@Z)

I included the Cereal library with the NuGet Package Manager.

As I am fairly new to programming and just learning about templates and more advanced libraries, I don't really know how to deal with this.