Hi,
If you had associated your Ec2 instance(assuming its about EC2 instance ) to an ElasticIP. That means there is no way for the IP to be changed, Because its a Static IP.
Also, Please let me know if you are accessing your Elastic IP directly or using an ALB Or you have associated your Elastic IP with a public domain name in Route 53?
I must say there should be something minor that you might have left. Hopefully, we can get that sorted may be in less than an hour.
BTW, I am an AWS certified Solutions architect and have good working experience with AWS in a billion dollar company. Hire me, if you think i would be the right person! :)