How to configure auto scaling group for bastion host.

I'm trying to build an auto-scaling group for the Bastion host. It has to get a fixed public IP when it is made a copy instance because of the available zone failover. Also, I'm trying to build an auto-scaling group for Nat-instance. I want to set the routing table that destination(0.0.0.0/0), Targets( Fixed network Interface in Nat-instance). Both 2 building's significant issues are that they can't use the same IP and network interface while autoscaling working. could you tell me how to set it to use the same IP and the same or fixed network interface while autoscaling processing?