The first thing to do is to download and install NoMachine on to the computer you want to access. It's free for personal use and allows one incoming connection. Then download the NoMachine app for iPhone/iPad available in the Apple app store.

To reach your computer over the Internet, the best way is to add it to NoMachine Network and generate its Machine Id. Then, from your iOS device, you can start a connection using that Machine Id. It's free to add a computer to NoMachine Network, but you will need a Network Subscription to connect using a Machine Id. Alternatively, you can connect using the IP address which is completely free to do from any NoMachine client.

For more details on how to get started, check out the following tutorials:

https://www.nomachine.com/getting-started-with-nomachine
https://www.nomachine.com/getting-started-with-nomachine-network
https://www.nomachine.com/getting-started-with-nomachine-for-ios