after rebooting my instance my IP address changes

The auto-assigned public IP address associated with EC2 instance changes every time you stop and start the instance.

An Elastic IP address is a static public IPv4 address associated with your AWS account in a specific Region. Unlike an auto-assigned public IP address, an Elastic IP address is preserved after you stop and start your instance in a virtual private cloud (VPC).

https://aws.amazon.com/premiumsupport/knowledge-center/ec2-recover-ip-address/