I want to use an additional Public IP for PPTPD VPN

Hello.
A secondary ENI must be attached to EC2 (Ubuntu).
See the following document for instructions on how to create a secondary ENI.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/MultipleIP.html

After creating the secondary ENI, configure the Elastic IP for the secondary ENI as described in the following document.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html#using-instance-addressing-eips-associating

After attaching the secondary ENI to EC2, follow the instructions in the following document to make it available from the Ubuntu side.
https://repost.aws/knowledge-center/ec2-ubuntu-secondary-network-interface