How to avoid new public ipv4 charges on my EC2 instance when it shouldn't need one?

I have a very simple EC2 instance that does some data processing for me. I only ever access it via a browser through EC2 instance connect and there is never a need for anybody to contact it directly from the internet.

I've read through all the blog posts sent with the notification of new charges and have set all associated vpc subnets to not auto-assign a public ip4 address and have created an EC2 Instance Connect Endpoint, but when I restart my instance it always get a new ipv4 address assigned.

I'm by no means a networking expert, but I would have thought that following the instructions means my instance has no need of a publicly accessible ipv4 address? Am I missing something? If I have an ipv4 addressed forced on me this would nearly double my current monthly charges and will make AWS untenable for my needs going forward.

Any help/explanation will be gratefully received.