You need to download the machine config which has the specific id_rsa for the machine that you want to access.
Wherever you launch the command should have access to the id_rsa. I haven’t tested in awhile, but I don’t ever remember doing a chmod for it.
What EC2 OS did you select? Sometimes you need to use different users based on your OS. For example, RancherOS would be ssh -i id_rsa rancher@<ip_of_host>, Ubuntu is ssh -i id_rsa ubuntu@<ip_of_host>.
Do you have more than 1 host? Please make sure you are in the correct host’s folder. If you have navigate inside the folder of the host, then you should be able to do ssh -i id_rsa ubuntu@54.152.228.146 and log into the machine.