To switch from using testnetConnection to a custom RPC URL in @mysten/sui.js, you should initialize the JsonRpcProvider with a Connection object that specifies your custom URL instead. Here's an example of how you can achieve this in your project:

import { Connection, JsonRpcProvider } from '@mysten/sui.js';

const connection = new Connection({
  fullnode: 'YOUR_CUSTOM_RPC_URL',
});
const provider = new JsonRpcProvider(connection);

This code snippet sets up a connection to your custom RPC endpoint instead of the default testnet connection. Make sure to replace 'YOUR_CUSTOM_RPC_URL' with your actual RPC URL.