I've never had a problem using SSH to access AWS EC2 instances and usually don't even need to include the key pair. In this example my EC2 instance is in a Custom VPC yet also in a public subnet with an IGW. It took hours just to get to the point now where when I SSH it asks me for my key pair's password. I've never been asked for a password before during SSH and have read a lot on here that doesn't seem to apply.
The command I run that asks for the password is:
ssh "2018-key.pem" ec2-user@XXX.XXX.X.XXX
I then get the following prompt:
However, I can use the same key pair to launch and access public EC2 instances via SSH. Any help would be sincerely appreciated.