I followed all the steps given on the tutorial page of AWS to create a subdomain(https://aws.amazon.com/es/premiumsupport/knowledge-center/create-subdomain-route-53/) and I'm pretty sure I got everything right because the tutorial is pretty straight forward. For context, before this I setup a LAMP stack on the server linked with my main domain (example.com).
My question is how to upload and manage files on my subdomain (subdomain.example.com). I thought that all I needed to do was to create a new EC2 instance and link it with the "hosted zone" of my subdomain, and after that I could just upload files and it would work (like I did on my original instance of the main domain). But after many tries clearly I'm doing something wrong, because the page of my subdomain (subdomain.example.com) keeps appearing blank with just the text "This site can't be reached."
You say that you installed a LAMP stack on the instance, so presumably there is a web server listening on port 80.
To test this, first login to the instance via SSH, then try curl localhost to test the web server. If that fails, then there is a problem with your web server.
If it works, the you should check the Security Group associated with the Amazon EC2 instance. It should be allowing incoming traffic on port 80 from 0.0.0.0/0.
Next, obtain the Public IP address of the instance. In a browser on your own computer, try accessing the IP address, eg http://1.2.3.4. That should work if the Security Group has been correctly configured.
By the way, you should be using an Elastic IP address (EIP) for the EC2 instance, which is a 'static' IP address that does not change. You can create an EIP in the EC2 management console, then associate it with the instance. This prevents the Public IP address from changing if the instance is stopped.
Next, try accessing the instance via the domain name. If this does not work, then test the name resolution by using ping with your domain name. The Ping itself won't work, but it should display the IP address that is linked to that domain name. Make sure that the IP address matches the Public IP address you used in the previous step.
If no IP address is provided, then you are missing an A-Record in the hosted zone. You should create the A-Record in the hosted zone and provide it with the Public IP address of the instance.