How to setup a permanent storage volume for EC2 instance?
That 150GB disk that r5ad.xlarge comes with is known as ephemeral storage - its content is wiped when you stop the instance. If you need persistent storage you can use r5a.xlarge that doesn't have the ephemeral storage disk and simply give it a bigger EBS disk. 

Either you can increase the size of the root disk during instance creation or create another EBS and use that. EBS volumes are persistent, they don't go away when you stop the instance (they may be deleted if you terminate the instance though). 

If you don't see the previous content of EBS volume after a reboot make sure you're properly mounting it during boot (hint: /etc/fstab).

Hope that helps :)