Deployed truffle contract: Cannot create instance of <contract>; no code at address

Question

I deployed my contract at 0xa518a732606baae1c72490313cb7bb42c62c7a50. Now, what I try to do is to interact with my functions.. I've created an node.js application over web3 and truffle/contract but I can't access my smart contract(it is named Diploma). My error: UnhandledPromiseRejectionWarning: Error: Cannot create instance of Diploma; no code at address 0xa518a732606baae1c72490313cb7bb42c62c7a50

Also, I made this post for asking more things I don't know if I understand good..

This is what I tried:

const express = require("express");
const app = express();
const Web3 = require("web3");
const contract = require("@truffle/contract");
const artifact = require("./build/contracts/Diploma.json");

app.use(express.json());

if (typeof web3 !== "undefined") {
  var web3 = new Web3(web3.currentProvider);
} else {
  var web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:7545"));
}

// when I npm start my default provider is taken as Ganache. I suppose it's normal since my node
// can't see my Metamask, right?

const Diploma = contract(artifact);
Diploma.setProvider(web3.currentProvider);

app.listen(8082, async () => {
  const accounts = await web3.eth.getAccounts(); // if I console.log this, all my 10 ganache 
                                                 // accounts are shown 
  const balance = await web3.eth.getBalance(accounts[0], function(err, result) {
    console.log('result', result);
 })

  var contract_address = "0xa518A732606baae1C72490313Cb7bB42c62c7a50";
  Diploma.at(contract_address).then(function (instance) { // here is where I get the error
    console.log(instance);
  });

});

Also, I have one more question. If I make transactions which I want to persist on Ropsten using my smart contract methods, can I choose the transaction hash / address where my data to be stocked or something like this? And after to retrieve them from that address?

Let's say this are my method:

function method(bytes32 _checksum, uint _data) public {
        require(!myMapping[_checksum].isSet);

        myMapping[_checksum].isSet = true;
        myMapping[_checksum].timestamp = block.timestamp;
        myMapping[_checksum].data = _data;
        myMapping[_checksum].setBy = msg.sender;
       
        some Event for logs
}

function getDiploma(bytes32 _checksum) public view returns(uint, address) {
        return (myMapping[_checksum].data, myMapping[_checksum].setBy);
    }

I suppose it's pretty obvious what I try to make. If this is not an optimum method can you give my an advice of how can I implement something to work how I want to..? If this is possible.

I've already saw this post and this post and more others..