On AWS EC2 when you boot, hostname set something like ip-172-31-41-172.eu-central-1.compute.internal
To chaneg this, run
hostnamectl set-hostname --static HOSTNAME_HERE
Example
hostnamectl set-hostname --static server1.serverok.in
To disable change of hostname, run
echo "preserve_hostname: true" > /etc/cloud/cloud.cfg.d/99_hostname.cfg
Now reboot your server with
reboot
Back to hostname